If he's straight, you can't.
If he's gay, and you are positive, all you should have to do is get on the topic. If he transtitions easily, then you might just wanna be like-- damn i need to jack off.
Then he might suggest you two do it together. Or he may be like-- me too. and then all you need to say is-- let's do it!
hahah.
Hope all goes well!