MambyPamby, I wouldn't make it into an 'If you go, we're finished' ultimatum. I'd just make clear that IF he chooses to go (completely his choice) that he will need to isolate somewhere other than your home for 14 days on his return AND have a negative C19 test.
My partner of many years was out and about throughout lockdown. Easy for me to be strict, we don't live together. BUT I don't see him, unless it's through a window and won't until I deem it safe to do so. He thinks I'm nuts. I think he's daft for risking his life. BUT it's his life to do with as he wishes.