Wow--that doesn't sound fun. Who are they to you now? To me, he sounds like someone renting a room in the same house. The last time I was in that situation, I think I said "Hi!" or "Goodnight!" when appropriate, once shared my laundry detergent, and once borrowed his dish detergent. If that's your view, I would set boundaries on where he sleeps (not next to you), whether you cook for him (no), whether you do his laundry (no), etc. You are no longer partners, and he's not acting remotely like a friend. Being busy or away sounds good. None of this precludes being friendly! I was friendly with that renter. If it gets intolerable, don't forget the safety valve of staying at a hotel (shared assets!) with your son.
