Stay with him at all times. If he's protective of you, be protective of him. It's a two-way street. Obviously, avoid the advances of other men. I've had two girlfriends who have used their paranoia as an excuse to cheat. Both wanted a long-term relationship, but thinking they could get away with that means they were both scum to kick to the curb. I'm almost 30 and I've never even thought about cheating on partner, so never make assumptions about a guy unless you want to end up another cheating, Judas loser.
That's call paranoid and lack of self confident. I think he feels insecure. Totally agree with many suggestions above. You need to be there for him, beside him, totally can be reach regardless, report to him every single step and most of all just focus on him. Be careful not to look at any guy even by accident. I really do mean it. You have to sacrifice most of your time and energy to earn his security and trust in you. For me I like jealousy that the guy falls into, I think make me feel blessed that I don't have to so this job. Been there, done that and it's work. ^^