Hmmmm.... Well he's a pony, and quite a few ponies are little gits!
I'd say he's just doing what he wants. It sounds oh so familiar to me, as I've had so many ponies like that!!!
What you need to do is ride him on really strongly. Tell him you ARE going to get over this fence. If he slows down, give him a kick. Hard. If he stops, you have him turn him away in a tight enough circle, then aim him at it and give him a smack.
I may sound harsh, but some ponies just require that kick up the butt to get moving!
Also, you could get a lead over a fence, to give him some courage :)