It could be the move. Maybe it's emotional
And yes your Mr. Tabby is a senior cat so (and I know this is easier said than done) he should have at least annual checks with the vet or 6 months checks. They can also recommend vitamins if needed of course. My goddess Hilda is 16 and ever since she started on vitamins for senior cats she got a new lease on life, although she's always been a very healthy, strong cat. But she seemed renewed ever since. (vitamins under vet's advice)
Does he poop and pees normally?
And yes, you'll be asked what do you feed your kitty. Food can be a key factor in their health, prevents a lot of problems when the quality is good.
I know how devastating for the budget vet bills can be and it takes time to recover from them but sometimes I have to since they're a part of the family and I love them.