No matter how old you are your mum and dad are always your mum and dad.  If you are worrying about Christmas cards and gift tags I would suggest something along the lines of his deserving a happy christmas and how you will love him forever and/or thanks for being such a great dad.  But I am sure you will know what is most appropriate from your lifetime together.  Yes, it does hurt - I know from my own experience of losing my parents.  Even though you know logically that your parents are likely to die before us it doesn't make it any easier when it happens.  Just tell him what a great dad he has been and is - talk about particularly happy memories and how you will never forget them.  Sorry, I feel that I am being pushy by suggesting things but I know how difficult it is.  I played it by ear when my dad died - took my lead from him when he was still able to talk to me and jusst said what was in my heart when it was just me speaking.  I am sure he will know how much you love him; somehow we find the words.