What you are describing is a sign of IBD or another chronic gastro-intestinal issue. A full blood panel (not just a CBC) should be run. Especially check the values for Amylase, Lipase, Albumin, Protein and the immune system markers. Chronic digestive issues can very quickly become detrimental, so I wouldn't hesitate on this based on his age. IMO bloodwork should always be the first thing a vet does as an xray is only ever going to give limited information. If you vet didn't suggest this, just did an xray and prescribed meds (which may or may not be what he needs), I would be looking for another vet.
As far as a diet goes, I would suggest a consultation with Susan Davis. She is a pet nutritionist who works with a holisitic vet in CA (phone consultations are what she does regularly). She has helped many of our yorkies on YT. At the very least, with consistent tummy issues, your baby should be on a daily probiotic supplement. Susan's information is invaluable and definitely worth the $....
Pet Health Pet Nutritionist Holistic Pet