Ask an experienced rider and/or instructor to help. Make sure that there are no medical issues. Maybe the horse doesn’t enjoy jumping and you should try dressage or a different discipline
I would try to find the reason why he does that, if health problems are out of the picture, If he's fit and working well on the flat work, then I would practice a lot of gimnastics and every time he does that, there has to be a consequence!
Something that worked for my horse was having the jumps small enough that even when se stopped I could make her go over them without doing a new approach, eventually she learned that it was easier to just jump them the first try. Getting a trainer to help might be a good idea!
