Hmm... never encountered this so I may well be wrong but I'll put down a few thoughts:
- Have you tried increasing the spread of the jump? If you make a nice big spread then he will (hopefully) have to jump it properly regardless of the height. You can then *hope* that he will start taking every jump seriously!
- Have you got any fillers? This may help to make it look a little more like an XC course and hence will mean solid jumps that he has to jump properly,
- "Tight turn" jumps. I'm aware that as he is quite young you cannot do too many of these but you could try challenging him with jumps that have a limited run-up so that he is slightly off-stride with the result that he has to jump it to clear it. This could just mean a nasty refusal though which is worse... not sure.
Of course it could all just mean that he has considerable talent that needs challenging with higher jumps to keep him happy! In that case I would just keep him going until he is able to tackle the higher levels of competition. It will be then that he realises that he has to try or he will end up flying in to a jump rather than over!