start of at the hight you normally jump and then using the same jump go slightly higher each time but dont just go from small to big set your horse up right and do it gradualy try not to make it obv to him.if this doesnt work try to lunge or loose jump him over it.

First of all, you haven't said how experienced you and he are? Also, if you have found that your horse is consistently refusing larger jumps, you might want to get him checked over. Perhaps he has had a bad experience in the past with jumping, in which case you should start small anyway, and build to larger heights. Good jumping really isn't about who can get over the highest, its about who is the most in tune with their horse, and who is riding the quietest and softest.

Good luck hope that helped!