We’ve had many, many cats. Quite a few of them, once they reach around 10 years old, seem to develop tummies that can’t take a lot of food in one go. Feed them first thing, they clear their dish and within 10 minutes throw it all back pretty-as it went down, undigested. We then feed them smaller portions and more on demand than scheduled. Glad to see you’re getting it checked out, we always do / did if it persists for more than a week or so, but my experience is that it’s often just the thing we call ‘getting on a bit’. Hope he’s fine, it’s always a worry.