You don't mention if you are using english or western tack, what kinds of pads and girths, etc. but there could be a couple of things going on.
First, a basic health check by a vet is always a good start, he can check for sensitive areas by palpating the horse. No big deal.
Second, your saddle should be fitted by a professional to ensure that it isn't pinching anywhere or applying too much pressure in one area. Many saddles need adjusting.
Third, you may want to try a gel filled girth, I used one for my daughter's thoroughbred as they tend to be sensitive.
Fourth, do up the girth very slowly, walk him a few steps, do it up some more.
Make sure that he is not bloated with food or water before tacking.
Lastly, search out an equine acupuncturist or equine physiotherapy specialist, they are well worth it.
Good Luck with this, don't give up