Its entirely possible that he could maintain erections throughout transition however its just as likely he will not be able to achieve a useful erection. All of that depends on his personal physiology.
If some one came to the TS support group I occasionally attend I would tell them not to plan on being able to have a useful erection, I don't like to blow pink smoke up anyones A$$. I think a person considering transition should expect the worse. I can still get an erection when I masturbate but I rarely get one during sex but then again I pretty much keep my penis out of the equation during sex.
