I taught him the C major scale HS and taught him how to read the treble clef. He understands well and has no problems in that, but I don't think he was really enjoying it.
Suggestions are welcome...
I wonder why he was not enjoying it....

First lesson: Tell him to play whatever he wants. Make noise. Play with fists, with feet, with his bum (Vitor Borge does it to great effect on the Hungarian Rhapsody). Show him how to do it - you do not wnat to get labelled as as solemn, stuffy teacher, do you?

If your uncle finds it strange, tell him you are teaching him modern music (e.g. Finnissy)
Then when he gets tired of making noise, tell him to play anything he wants with his fingers, as long as it is on the black keys (it will always sound good). From time to time do some black key improvisation so that he has a model to aim for.
If you have time (a first lesson should not last more than 15 minutes), teach him a simple duet (e.g. "The Chinese Clock"; "Blue moon") and play four hands with him.
His home assignment: Bring for the next lesson a list of pieces he would love to be able to play. At the end of the lesson you can show him some really easy but nice pieces. If he likes any....
Second lesson - teach him by rote (you play, he imitates) the easy nice piece he selected from the ones you showed him the previous lesson. Have a look at his list to see if there is anything appropriate he can learn on the third lesson.
Do more improvisations/duets.
Third lesson: Start working on one piece of his list. Use the piece to teach about sight-reading, scales and so on (the pragmatical method) rather than what you are doing (the logical method - teaching a C major scale for which there is no musical context - that is, a reason for doing it he can understand).
By then he should be hooked.

And of course read all of this stuff:
https://pianoforum.net/smf/index.php/topic,587.msg14335.html#msg14335(How to teach a 5-year old to read music – Candida Tobin)
https://pianoforum.net/smf/index.php/topic,1884.msg14708.html#msg14708(Motivating children – joining the child’s model and using other children’s accomplishments to motivate)
https://pianoforum.net/smf/index.php/topic,2047.msg16848.html#msg16848(what to do with children crying)
https://pianoforum.net/smf/index.php/topic,2077.msg17179.html#msg17179(How to become a teacher)
https://pianoforum.net/smf/index.php/topic,2115.msg17860.html#msg17860(the advantages of a teacher – comparison with typing)
https://pianoforum.net/smf/index.php/topic,2125.msg17864.html#msg17864(Age limitations – Barenboins’s there are only bad teachers philosophy)
https://pianoforum.net/smf/index.php/topic,2192.msg18542.html#msg18542(methods for very young students – Candida website)
https://pianoforum.net/smf/index.php/topic,2246.msg19041.html#msg19041(Young teachers – why young/bad teachers are not a problem – When the student is ready the right teacher will appear)
https://pianoforum.net/smf/index.php/topic,2260.msg19270.html#msg19270(Dear Bernhard thread – Pieces leading up to the revolutionary)
https://pianoforum.net/smf/index.php/topic,2032.msg19339.html#msg19339(How to become a piano teacher – good advice from minsmusic – the pitfalls as well as the requirements)
https://pianoforum.net/smf/index.php/topic,2273.msg19432.html#msg19432(what to do when a student has not practised)
https://pianoforum.net/smf/index.php/topic,2192.msg21823.html#msg21823(How to teach very young students – the historical method, the pragmatical x logical method and total exposure as the best way for under-5s)
https://pianoforum.net/smf/index.php/topic,2907.msg25589.html#msg25589(how to teach chords to young children)
https://pianoforum.net/smf/index.php/topic,2906.msg25591.html#msg25591(How to organise a lesson syllabus – analogy with cooking)
https://pianoforum.net/smf/index.php/topic,2970.msg25991.html#msg25991(Areas of study for a complete music syllabus – books for each area)
https://pianoforum.net/smf/index.php/topic,2979.msg26080.html#msg26080(Methods)
https://pianoforum.net/smf/index.php/topic,2994.msg26162.html#msg26162(Teaching little brats – recorder rules)
https://pianoforum.net/smf/index.php/topic,2972.msg26178.html#msg26178(advantages and disadvantages of teaching by rote – good for the teacher)
https://pianoforum.net/smf/index.php/topic,3014.msg26427.html#msg26427(How to deal with bad students – using the student for experiments)
https://pianoforum.net/smf/index.php/topic,3066.msg26951.html#msg26951(Teaching or playing – learning with the student)
https://pianoforum.net/smf/index.php/topic,3442.msg30359.html#msg30359(How to choose your next piece – impossible/easy pieces)
https://pianoforum.net/smf/index.php/topic,3524.msg32403.html#msg32403(adults learn faster than children)
https://pianoforum.net/smf/index.php/topic,3783.msg34200.html#msg34200(how to teach super motivated students)
https://pianoforum.net/smf/index.php/topic,143.msg35967.html#msg35967(differences in learning/teaching children and adults)
https://pianoforum.net/smf/index.php/topic,3891.msg35968.html#msg35968(Types of students. Good and bad attitudes displayed by students)
https://pianoforum.net/smf/index.php/topic,4020.msg36635.html#msg36635(What to teach a 4-year old in his first lesson)
https://pianoforum.net/smf/index.php/topic,4269.msg39706.html#msg39706(Helping a student get rid of bad habits)
https://pianoforum.net/smf/index.php/topic,4336.msg40366.html#msg40366(handling transfer students – the 3 most important areas in piano teaching)
https://pianoforum.net/smf/index.php/topic,4321.msg40373.html#msg40373(why do you teach? – policies and teaching methods –pragmatical theory of memory)
https://pianoforum.net/smf/index.php/topic,4405.msg40979.html#msg40979(method books)
https://pianoforum.net/smf/index.php/topic,4417.msg41108.html#msg41108(scolding students – students from hell, their attitudes and appropriate responses – start from the idea that student is put on your path for your benefit, not his – joining the model of the student – using their energy rather than yours – analogy to martial arts – use students form hell to learn this art – logic x psychology x philosophy – example of learning piece backwards- steve wonder is god silogism – analogy with reading)
https://pianoforum.net/smf/index.php/topic,4269.msg41284.html#msg41284(teaching a child with bad habits – aim is not for the student to get better, but for the teacher to get better – use pieces the student love – outlining x simplification – how small hands can cope with octaves – fingering)
https://pianoforum.net/smf/index.php/topic,4002.msg46084.html#msg46084(m1469 describes the philosophy of a Russian teacher – the pragmatic method and doing wrist locks on students)

Best wishes,
Bernhard.