I went through a phase like this. I'd say a good test is to let him fantasize and see where his mind takes him, and keep note of the results. After a big enough sample size, see what % he gets off more to.

I also am pretty sure if pegging doesn't satiate his desires, he's likely bi or gay. Also, if he can't get off to a picture of a man in model like pics, he's probably not gay. If he doesn't find men attractive in person, he's probably not gay. If he doesn't like the idea of a romantic date with a guy and kissing him and stuff, he's probably not gay.

Really, he probably needs to suck and bang a real one and see how that goes. I realized the reality of bottoming was much different than the fantasy, in a bad way. So that fantasy got killed. I also though sucking dick sounded hot, but that made me sore, tasted bad, and really just enjoyable. So that fantasy died too. What did I have left? Not much, considering I've never gotten off to a picture of a guy, or even considered one attractive in public. Occasionally I'll top one if I'm seeing red and can't find a girl to bang, but that's a last resort. I really just lost the desire for dick when the reality killed the fantasy.

I guess I like the power dynamics involved, my first gf called me small, so I think a penis fetish developed as a form of small penis humiliation. Having another, bigger dick immediately in my vicinity reminded me of my smallness and my "traumatic event" which I was re-visiting.

The sexualized insecurity is at the core of it, and I realized I can still get that feeling from pegging, even more so because the girl can say stuff like "I'm a girl and mine is bigger." And you get the same sensations of dick without the shitty taste, plus the person wielding a plastic one is usually a girl and usually more sensual.

So that's why I say if he can't be satisfied by pegging, probably bi or gay.