there is no positive way to ask for someones private information. I dont know who your trying to fool here, we all know why you wanted his password.. your only lying to yourself, you even mentioned his past relationship. Your insecurity is going to ruin any relationship you have or will have if you continue asking for stuff like a facebook password, or any password for that matter.

I caught a girlfriend looking through my phone once and that was it, I never trusted her again. It just totally shows your hand, you proved that you dont trust him and now your probably more curious and concerned then before.

Not to mention, usually people get infidelity concerns because of the things they themselves are doing. Just be thankful he at least had a pair large enough to tell you no. Now you just wont know about his next account.. have fun with that