Well a dog with cancer, I am a bit surprised that the vet. did not suggest a diet. Since the inability to digest certain food is due to a serious health issue I would absolutely contact a vet. if my vet couldnt help, I would contact a teaching vet. hospital and see what they had to say and maybe get a refferal to an oncoligist that can help in that matter. Feeding a cancer sick dog is a lot different than feeding a healthy dog and it also depends on the cancer type and other side effects the cancer may have given.
I hope the poor guy, I hope they find out a diet he can eat and keep the food down. Send him my huggs and best wishes.
Marit