There is no way to know whether your dude is trustworthy by our perspective but... based on the fact that he's never cheated, that, is what you should go by. Trust him based on reality, don't be suspicious of him because of your fears. that will surely put a wedge between you.

As for people who will cheat given the opportunity, let me take a moment to shatter your view. I am over 40, been married 2x and neither break up was ever due to my own infidelity. Marriages sucked that I was in but in no way did that ever give me an excuse to cheat. Was I given opportunity? Sure just like every other person on this earth we are faced with that every day. I can positively say if it were me, I wouldn't be in his position to go to a party without my SO but.. if I were, that isn't any different than every day life. there are women who would throw themselves at you and men that would take them up on it both attached and unattached but that does not say that it matters to me. when I am taken that's it. done. lock, stock and barrel. Sorry lady you can't have me, in other words. I'd like to believe I'm not all that unique in this and it sounds like you may have someone with the same views.

Trust when given reason to. Only let real events break that trust. I believe from what you've said you have no reason to not trust him but again that is based one what you've said here.