First, get him used to a jumping position. Walk, trot, and canter while maintaining a correct jumping position. I'd suggest doing walk, trot and canter over poles, slightly raising them over the days/weeks as you both grow confident. Then I'd try lunging him over a tiny crossrail, just so you can see how he will jump it. I would NOT suggest seeing how high he can jump at this point because this is very new to him and you wouldn't want him to hurt himself. Next, I'd get on and walk up to the tiny crossrail and let him sniff it out. Once you both feel confident enough, try walking over it. Release with your hands as your going over but do not get into jumping position. It is important you stay relaxed and confident before and after the crossrail. Halt him a few strides after the jump. This teaches him not to run away from the fence. Once you both feel confident enough, you can try trotting it. Remember to stop a few strides after the jump you just trotted over. Be prepared for all sorts of ways he'd jump it (popping it, taking it really long, etc.)!
This will probably take days, maybe even weeks. It is will pay off in the long run to take it step-by-step. I'd do each step I just said one at a time and only take one step per day.Also, I cannot stress how important it is to stay relaxed, balanced, and confident for your horse! This will be a very new and stressful thing for him and the expirience will be ten times worse if you are not relaxed, balanced, and confident. Make sure to reward him alot too!
Hope this helps