I never gave anything to my therapists but would be quite surprised if anyone declined a card. I can hardly imagine anything more professional than that. If you are worried that he will infuse it with something, maybe choose a simple and traditional card and, if you plan to write a message on it, come up with something appreciative but brief and relatively formal. And I agree with DP's suggestion about giving it to him at the end of the session. I would personally hand it to him directly and would wish him happy Holidays/Christmas but leaving it on his desk might also be good.