Is it around the girth area? If so, I would suspect some damage to the pectorals and lay off riding for a while to let him heal up. Try gently massaging his sides with a hand massager (one of those knobbly things you can get in the chemist) or just your knuckles. Use light pressure and little circular movements to release any tension in the muscles. You can also try doing some stretches with his legs to loosen the muscles- pick up his front legs and stretch them forwards slowly, holding around his pastern, hold for 5 seconds, then let him put it down. pick up his back leg and stretch it forwards slowly, like the farrier does to put the foot on a stand to tidy up, holding around his pastern, hold for 5 seconds, then let him put it down. You will find he starts to relax into it and will be able to stretch further over time. Make sure you only stretch the legs forwards though, not out to the side.
A visit from the physio may well be a good idea, also check your worming plan and see if he's due. A high burden can cause colic-like pain.