My little QH did hunters primarily, but also jumpers, eq, and fox hunting before he started eventing. since he had such a strong background in SJ it was very easy for us to move up places there and he always rode SJ the way he rode jumpers. more pace than hunters of course but never skipped a beat.
Some horses may not deal as well with cross training - I took my mare to a completely weenie (as in total of 5 horses and no real runs) fox hunt this fall and the next time we went out 'officially' - to a schooling show, she was a complete raving lunatic from the minute I got her off the trailer. WE'RE GOING HUNTING!!!!!!!! Let's just say I cleared the warm up trying to canter and we did our jump rounds after 1 cross rail. She did figure out as we did the first fence, that it was just plain old boring show jumping, but she sure was a pill until that. But give it a try, he might be just fine!
NTolppa - Definitely. I plan to supplement both my horses' season in 2013 with jumper classes. Both my boys are young (coming 4 and 5), and aside from the jump practice, it's an opportunity to get them out in a show environment with the flexibility of doing just 1 or more classes. It's great practice for both of us and a wonderful way to school jump questions over quality courses and footing.