Should I tell him? And if I should, how should I?

He's going to find out sooner or later, life has a way of making sure stuff like this comes out. If you don't just tell him about it right now, he's going to feel hurt about you hiding things from him. He may feel like your'e hiding more than just this & you truly may lose him for good. Just be honest. He probably just wants you to be happy and safe. Reure him that you CAN tell him everything and you're working on your problems instead of keeping them all bottled up