Wheaten terrier digestive issues?

Our 10-year-old Wheaten has recently developed digestive problems (nausea, vomiting, diarrhea). He’s been investigated for all sorts of health issues, but his tests always come back normal. A year ago he started on a prescription “joint diet” for mild osteoarthritis. The main ingredients in this food are corn, chicken liver, fish oil and wheat (with various other ingredients). He was previously on two different chicken-and-brown-rice foods, which appeared not to cause problems.

We have found throughout his life that he has a sensitive digestion, with specific proteins (e.g., beef) and higher-fat foods causing discomfort.

Has anyone had experience with their dog developing food sensitivities a year or more after starting a food? And has anyone ever had a dog with food issues who didn’t have skin problems? Our dog doesn’t get itchy, and is otherwise very healthy.

Thanks.