Rach, sorry about your cat.
Re. the mate/boyfriend problem. Your mate obviously knows that you are about to move in with your B/F. If he fancied you in first year my bet is that he never stopped and basically he feels that it is now or never given your situation with your boyfriend.
He has probably been hanging on hoping that things don't work out for you with your B/F and that he would be around to pick up the pieces...
I would tell him firmly that there is no hope and that it is mates or nothing. He may well go for the nothing option.