Well, there's currently no evidence that those two are connected, despite hundreds of studies on the subject. As of right now, there's no reason to believe either gender or sexuality can be determined by influence (this is also why all that wishful thinking about strong fathers preventing kids from being gay or trans is doomed to fail.) It also doesn't make logical sense- gender and sexuality aren't the same thing, nor are they two sides of the same coin. Being homosexual is not the same thing as wanting to be or acting like a member of the opposite sex. They are both very different kinds of phenomena. If the evidence changes, though, I'll change my position.
It's also very possible he was born that way. Remember, most people who end up as transgenders as adults exhibit the associated characteristics before puberty, when they are 5-10 years old- before sexual development, before they know what sex is, and before they are even fully aware of gender roles. For someone to exhibit transgender traits that early, without the relevant knowledge for it to be a decision influenced by others, strongly suggests that many of them are born with it, due to chemical or epigenetic factors.
A lot of what I'm saying comes from official sources like this one:
http://www.apa.org/topics/sexuality/transgender.aspx