You might have him checked over by a chiropratcor, and have him/her check your saddle fit and his jaw/poll area. Have you dicussed this with your vet? Could be some discomfort due to teeth needing floating.
I do agree with Kevin though, usually bit chewing is behavioral, not because of physical pain. I do like to rule out physical pain first though, as it's easier to deal with than training or psyche issues. If he checks out physically (and don't just assume he's fine, get him looked at!), then I would spend more time doing ground work with him and bonding. I'd also keep things to flat work if you're jumping him. Work on your Dressage work and some trail rides.
I would also look at his diet and turnout. Feed can give your horse too much energy, which can manifest negatively. What kind of feed does he get (exact brand/type), how much, and how often? What kind of supplements does he get? What kind of hay does he get, how much, and how often? How much turnout does he get? Any grazing? Is he turned out alone or with friends? How much room does he have? How big is his stall? Do you have any stall toys?
A simple change in diet and/or turnout can make a big difference!