What should I do?

i think you already extended an invitation and he turned it down. don't let him guilt you he had an opportunity and plenty of chances to be there for you. there are other people who already were invited and they deserve to go just as much. if he really cared about going he could sit in the extra seating. it's not your fault there were chances and he chose to not be there. don't stress if he changes his mind he'll take what he can get. he should have tried to visit, or call or be some sort of support when he had a chance, not just show up whenever it's conveinent, and expect to get priority over people who have been there all along. good luck and congratulations!!