You can't fix that during a competition. You must fix it at home. Trotting or canter poles AFTER your jump will encourage him to slow down, focus, and watch his feet rather than play around.
Make sure you have your leg on after the fence.
Sit up and sit deep, put your leg on and ignore the behaviour unless it becomes dangerous. Focus on you goal of getting to the next jump. If you have a crop, try giving him a good tap on his shoulder to remind him you are there