He is more susceptible to pancreatitis once he has had an issue with it. My guess would be he is dealing with a mild case of it, which could progress to a full blown case. The worst thing we can do for our dogs who are not feeling well is to continue to feed them and/or coax them to eat by giving enticing, but fatty or unfamiliar things which further exacerbate the problem.
You need to give his stomach, pancreas, gut time to calm down - at minimum a 24 hour fast and that means NOTHING by mouth for 24 hours sans water. Then start back with a very bland diet, boiled chicken/rice - four VERY small meals during the day x three days, then slowly add his regular kibble (ditch the holistic food) to the bland diet x three days and work your way back up to his normal feeding.
Once back on track I would recommend you stick with his regular diet and NEVER switch to a new food. His pancreatitis issues make him susceptible to flare-ups; the older he gets, the more susceptible. If after 24 fast and 1 day of feeding small, frequent meals he seems to regress, get to a vet. Or if he seems worse with a continued painful belly, prior to trying the fast, get to a vet. Pancreatitis can kill and should not be taken lightly.