My on/off boyfriend of 3 years had a one night stand with somebody a few months ago when we were in a break. He's assured me no contact since then but is now on social media dishing the likes out. I've told him I've seen this and he's lost his temper with me for checking on him. He's 50 years old not a boy. clearly I don't trust him and for good reason it now seems. But I don't know how to discuss this with him as he's turned it around as a me problem.