I would keep him on the flat until you can get him loosened up. Take some Dressage lessons over the winter and get your flat work 100%. As he gets looser, have your Dressage instructor add in some trot poles and cavelleties (sp?). This will help him relax trotting over things, which should also help him over fences.
When you get back to your fence work, use your new flat work to collect him before and after the fence. When he rushes at a fence, collect him more, transition down, or (if you have enough room) turn and circle.
Using gymnastics also helps with a horse that rushes. Set up a long line of small obstacles, like a canter pole to a cross-rail, bounce to a small 1' or 1'6" vertical, one stride to a cross-rail, bounce to another cross-rail, with a canter pole at the end. These kinds of gymnastics will force your horse to slow down and think about the obstacles. BUT, it is very important NOT to put a horse that is a bad rusher through a gymnastic right off. If you do, you risk a bad wreck in the middle... You need to go back a few steps and get your flat work down, so you can control him within the gymnastic.
A new bit can help, but training is really what your horse needs. I personally would use a kimberwick with a roller in the middle. That would give him a mouth piece like what he's used to for flat work, but a bit more brakes. Like this one: