Westie with sensitive tummy

I have a Westie who is 4 and has always had tummy problems with vomitting and loose stools and has been put on a diet of Royal Canin Canine Hypoallergenic and pepcid tablets by the vet although my dog still vomitts on occasions, say every month and regularly doesn't eat his food but when he does he pushes it around the bowl and it takes him a while to eat it. I think this could be due to the fact he doesn't like it and wanted to know if anyone knew of a similar food which he would be able to eat. Other than vomitting he is a very happy wee dog and has a good coat and skin.