You might surprised how spacious half a queen bed can be, and he looks to be fit and trim overall. I don't think he's likely to have much trouble.
I shared a 20" wide camping mattress in a pinch. We are both very slender. Spooning was mandatory.
Oh. Well. Yeah, I can see how that's a real problem.
(Actually, I spent almost a decade married, so, gentle sarcasm aside, I can see how that might be a problem. On the other hand, I get the impression the cases aren't really parallel. I mean, if it were me and I had someone moving in, it'd be a good occasion for a rebuild to meet current needs, I think.)