The lipomas probably look even bigger because he's lost weight!

Usually lipomas are harmless, but since it's affecting his breathing it sounds like it's time to have them removed. They can come back if you don't get all the cells out - and sometimes it's hard to get them all. But I don't know about radiation . . . it seems to me that it would be worth trying to just remove the lipoma that's affecting the breathing, and then see if it returns. If it doesn't return or grows back very slowly then maybe your dog won't have to undergo radiation.

Good luck!