If it is low enough make him walk or trot over it when he stops. Make the decision uncomfortable for him/her to stop. Also make sure that you are not pulling on his/her mouth and keep constant leg pressure on him/her
Some horses do not enjoy jumping. Perhaps he just isn't a fan of jumping. I rode a paint mare loved western gaming, took her over jumps and she hardly put effort into it and took her to a jumping clinic with a show jumper and she threw me over almost over all the jumps each time (before this she was okay with jumping and thought it would help but nope she just didn't enjoy it)
How long have you been jumping this horse? Have you been doing it on your own or in lessons? Just trying to gauge where your at. My horse used to be a notorious stopper and his was just a confidence issue with both him and I.
There are a few reasons why a horse refuses:
Rider error: the rider rips their body forward, leg back, heels up, chin down etc, putting the horse off balance.
They've been allowed to before: if the horse has refused before, they may have realized its a way out of work, and are going to keep trying it till they learn it isn't.
Bad past experiences: if a horse were to fall on a jump, injure them self jumping, spook at something other than the jump right before jumping, they may be put off jumping. In this case baby steps are needed, starting again with poles, then crossrails etc.
Being overfaced: if a jump is too big or scary for the horse, their fear of jumping it can be carried over to the next time you jump, putting them off jumping. Again, baby steps would be needed.