Start by working on your flat work until you and your horse can manage all transitions perfectly and with minimal aids from halt to walk to trot to canter and back again.  Then just put some poles out and practise halt, walk and trot transitions over them.  Put up some mini jumps (nothing higher than 2ft) and do the same until he is relaxed and listening to you throughout.  Obviously, not all this will be in the same session.

Everyday before jumping you can start off by working on your transitions using mainly your seat and position instead of your hands.  When your horse in completely relaxed and responsive then do the same over poles again, and then work up to jumps.  You could try going back to walk immediately after each jump, and eventually he'll start anticipating that and slow down.  The second he is in walk, give him loads of praise.  That way, you shouldn't need to resort to a stronger bit.

A lot of the time, it is easier for a horse with weak abdominals to speed up after a jump because that way it is easier to rebalance themselves that way when they are heavy on their forehand.  Pole work and transitions will help with that.

Hope any of this helps.  Resorting to a stronger bit may help for a while, but eventually the horse usually gets used to it and you'll need an even stronger one, or worse case scenario, it will be uncomfortable for a horse that finds it very difficult to stop.  The trick is to train them really well in transitions, and then apply that to the jumping.

Good luck! Xx

You may have to get a slightly stronger bit for jumping in. Put poles either side of the jump so he has something to think about when he lands.