BUN can be affected among other things by what he ate, dehydration, liver problems, stress and if he lost weight a little bit suddenly and tends to be a little on the high side when a cat is having a high protein diet but it can certainly vary
Creatinine can also be affected by a big weight loss ( because cats tend to loose muscle mass when they loose since is related with the amount of muscle mass a cat has a cat with a lot of muscle would have higher creatinine levels than a small one ) or by dehydration
Both can be affected by infection, so there are a lot of issues in place has he had an SDMA test? this test would tell you more accurately if there's something wrong with his kidneys