Why did my boyfriend get upset over this, is this a red flag?

Maybe his mother is too intrusive and has ed her head in to his business in the past when she shouldn't have... But I mean, the dude is still living at home so he can't expect a whole lot of privacy. He could be worried that you and his mom will get together and she will tell you all his deep, dark secrets and embar the crap out of him or he could just be a giant douchecanoe!