Lots of transition work...get him/her to move forward off the hind end.
I'm having the exact same problem with my gelding and what is working for him is tying an elastic polo wrap around the girth and then extending around his hind end on top of his hocks and tie it onto the girth on the other side. I lunge him and ride him in this, not only does it give him a reminder about his hind legs but also helps him tuck his hind end under which allows more push from behind.
I think that poles work and cavalettis exercises could help you.
Yes, if he's not using his backend, whether it be due to laziness or lack of ability (regardless), he needs more time on the flat building up the muscle so he can actually use his hind over jumps. A horse needs to be able to correctly carry themselves on the flat before they can jump :)
I think you probably need to spend more time on his flatwork and hacking out to get him fit before jumping him too much and then concentrate on the basics, lots of groundwork.