It is impossible for us to say what is in his mind and heart on the issue, but there are certainly many, many straight identifying men who watch gay porn just as there are many, many straight identifying women who watch lesbian porn. The latter statistic has been proven by scientific data, though the former is hard to quantify due to the nature of stigma attached with men being sexually fluid to some degree (whereas it is almost expected of women).
Given that he is trans, things might get a little more complex on the issue, as I know it has certainly clouded my view of gender attraction, as when it comes to women I honestly don't know what is natural attraction and what is socialization and male hormones.
I'd say give him the benefit of the doubt, and be aware of and accept that it may very well just be simple fantasy. But if you truly have doubts, speak with him some more about the issue. Maybe he can offer something that sets your mind at ease if you show that it truly worries you.
