I would actually try to work with the habit rather than do away with it. Many students have sloppy rhythm because they are lazy to subdivide. Work with him in gaining acuity of the subdivisions. A solid understanding of how to work with subdivisions and micro-pulses in music can lead to better development of beat flexibility, purposeful applications of rubato, and heightened command of expression. It is not the notes, but what we do in between the notes that generates effective music making. You have a student that seems to possess a sense of this. Help him develop the skill so it assists his music making, but in a way that mannerisms do not get in the way of music making.
Okay, he's playing a few pieces. Bartok - Mikrokosmos #75 (Sometimes crushes last triplet notes, and sometimes plays even quavers slightly swung)Mozart - Sonata in A, K331 - Theme and Var 1 only (Var semiquavers are almost steady, except for rare moments when co-ordination is hard and the trill trips him up)Bach - Prelude in c minor (broken chord prelude. Has slightly stiffness in LH when notes get quite far apart in bass, trying to work on it. RH is almost perfectly steady with semiquavers)Schumann - Wild Horseman (no problem with steady quavers, hands are a little tense but not too bad) Tchaikovsky - Morning Prayer (Dotted quaver and semiquaver patters are a little swing like, and not quite perfectly in time)@quantum... Problem is, one he was doing a piece with crotchets and he was counting the quaver beats, however it wasn't:1 + 2 + 3 + it was more like:1+ 2+ 3+ I've been using an app 'ReadRhythm' on the iPad to really help him try and feel and at least tap the rhythm to get a sense of evenness. I've been trying to focus on specific rhythms - making sure quavers are very even, then quaver-crotchet combos (syncopation), dotted quaver-semiquavers to make sure they're very tight and rhythmically secure. The problem is - the mannerism is SO linked in with what he plays and he's done this for years, even before I taught him... so trying to undo it, is the problem I'm having... HOW does one try and ensure the mannerism does not distract from the playing?@anamnesis... Interesting point, but I believe that just simple verbalisation can help. Have a student who is having trouble with syncopated rhythms? I get them to say the rhythm names for a while, tap it and say it, then get back to playing without saying, if they're confident. It helps with their understanding of the rhythm of the passage.
Just based on your write-up, your student is relating the upbeat notes to the preceding note rather than forward, which is what is distorting the rhythm.