If you think think alcohol is his thing, then I’d get him a nice bottle of scotch over a larger amount of something cheaper. He’ll remember the best scotch he’s ever had for a long time, while just more beer is going to be forgotten.

If he objects to you purchasing items for him - and that I understand - I highly recommend a home baked item, personally decorated to celebrate his time taking care of your beloved cat. I’m betting he’ll like your appreciation more than the value of a gift, and that’s a delicious way to show it.

If he is the kind of person who send pictures of him and your cat while youre away, have a small one printed and framed and gift it to him with a hand written thank you.