Check out those links above for help.

You’ll have to get records from your vet and you may have to update your cat’s shots (especially rabies). If he’s not microchipped, that will probably have to be done as well.

Check with your airline, but you may be able to carry him on board the plane with you, instead of having him placed below.

Poland will likely have some sort of isolation policy - a time period (could be up to several months) where he’s not allowed to be near other animals. Presumably, any health problems will have shown up by then and he won’t have infected any other animals. Luckily, since you’re moving to Poland, and not just visiting, there’s ample time for that
Before you leave, check and double check that you have all the proper documentation for your cat, the approved carrier for the airline, etc. Get a copy of written policies that you can carry with you.

I don’t blame you for being so in love with him that you can’t part - I’m very much attached to all of my animals! I hope you have a safe trip and good luck in Poland!