Looking for an older title. Son is getting harassed at school because he doesn't have a GF so the mom pretends to be his GF, this leads into them having to act like BF/GF and eventually the friends think he's just paying some women so he convinces him mom to be his GF again when they come over. It ends with the two friends having sex with his mom while he watches.
Sounds a bit like URE-073, but the son joins in with his two friends at the end.