If it's not happening consistently, you're worrying about nothing. I personally find bad hygeine to be very deflating, I just can't hang with stank or skank. Back or knee pain can cause a stiffy to vanish quickly, so if you just finished having him move furniture or he's been under the Buick all afternoon, you might want to be the one on top.
But another factor that can really douse the flame is ridicule/bitching. Seriously, if you tend to get critical of your mate during sex, don't expect peak performance unless he happens to prefer grudge fucking you for running your mouth. There are only a few REAL issues that can limit erectile performance, the rest are mental. Age is only a marginal factor, en Franklin fathered children into his 80s.
By the way, wtf is this "I didn't give it up for 3 months" business? Can you possibly make sex seem less romantic than phrasing it like it's some kind of commodity? As if it's SO valuable it should have a stock ticker symbol? I'm not suggesting that anyone should have sex before they are ready, but this sounds like a series of qualifying stages or landmarks must be met before you, the owner of sex, can allow such contact. It sounds so cold and calculating and I have to say that itself is a serious turn-off once a guy realizes he has been manipulated for so long. Unless, of course, you are VERY talented and 3 months is worth the wait.
So much about this is a matter of perception. Making a guy jump through hoops is not how a lasting, loving relationship happens. It's no better than keeping score, as if it's all a game to you. Good luck with that.