A tailor can't really help you. If it were a slight adjustment, you might have the collar button moved, but more than a fraction and it will throw the placket off-center. Tightening your tie will simply bunch the collar. Bite the bullet and buy shirts in your correct size. But I would advise you to hold on to your old shirts. Men's necks get bigger as they get older, and if you live long enough you'll wear those shirts again. It's a fact of life and doesn't have anything to do with weight gain. I wear a full size bigger than I did in college, and I'm basically the same weight. I think carrying your head around is like lifting weights or something.
