Has he always been like this and you've reached your limit? Or did it used to be better and has gotten worse? There's introversion and then there's not leaving the house.

It has definitely gotten worse. In fact, now that I'm thinking about it, immediately family almost never leaves the house to the point of appearing agoraphobic.

He says "going out to dinner kind of stresses him out because of the noise and people." How loud do the restaurants have to be for that to trigger him? (Some restaurants are noisier than others.)

If the restaurant is more than half full it is too much. We live in a metropolitan area where there's no such thing as a quiet restaurant unless it's 2am at Denny's. Or a 5 star joint. I don't like noisy places either but it's just a reality of where we live.

Has he talked to a therapist about his increasing desire not to leave the house?

No, I'm not sure he sees it as a problem. Although I haven't specifically told him that from the outside it is starting to appear abnormal. He sees his mom never leaving the house as abnormal, I might have to approach it from that perspective. I worry he's turning into her.