My horse is of a similar build, I think, and she's taking a long time to work out the canter too!
What has helped:
Equipment: a sheepskin half-pad, a new saddle that sits higher over the withers. She's quite sensitive wither-wise, I think. You say your saddle fits well, but I think you might try setting the saddle a wee bit further back. He might be fussy around the withers too.
Riding: I was told by a prominant judge/clinician that bceause my horse is a mare, "you can't tell her anything!" and to learn how to ride the canter to keep her happy.
In general, especially at this stage, I fully agree-- so if he likes you sitting light, I'd keep that, and only gradually teach him it's okay for you take a deeper seat.
An exercise that made a difference: the bow tie, which is a sort of flat figure-8 along the fence. You trot the round parts, canter the straight parts, don't worry about leads, worry about getting back down to the trot for the loops. Anyway, it got her anticipating the canter, which was great (she tended to balk-and-buck) and actually enjoyed the routine. The exercise was told me by a barrel racer, as an exercise to SLOW a horse down, so you might try it.
I'm happy to say the canter itself is not the issue any more, we're working on staying through, round, and have begun doing all the exercises we've been doing at walk and trot.