Try riding with spurs, and give him a little poke with them a stride or two before the fence. Make sure you are relaxed and balanced, because if you are tense, he will be tense too. Make sure he has a long enough rein to jump and dont pull at all. Dont go into your jumping position until the front legs have left the ground so you will be less likely to fall off. Hope this helps :)
Do you use a crop with him because if you do one strid before the jump tap him on the shoulder the let him know to go. Also after the jump snap straight back to sitting in your canter like you havent done a jump.