people that play by ear are smart yet lazy. make them spend a certain portion of time sightreading (a small portion). make it unpainful and actually fun by just a few measures. add to it every day. the same measures - adding one more. teach them to look ahead more each time (move an index card for one or two chords ahead - after they read the first chord). tell them to scan what is ahead - RIGHT AFTER they play the first chord. then, when they have fully scanned the next chord and have the fingers on the notes to play it - tell them to scan the next one BEFORE they play the chord. sometimes this is confusing to first time players - but ultimately the goal is to scan an entire page before playing, so one is not surprised by what is on the page. then, as you go - at least a measure or two is taken in by the eyes at at time.
have him play at a steady VERY SLOW beat. this is what does most sight readers in. they don't want to play slowly and evenly. and, they don't practice what they need to to sight read.
maybe if you brought an electronic keyboard to practice on? a paper keyboard is sort of pointless.