Well the issue here is that you don't really have any control over when he tells his family. Even if he is supportive, understanding and happy to stay with you, through your transition he has become a gay man. Therefore telling his family is more than just telling them about you, it's effectively him coming out to his family. Did you come out to your parents straight away? Did you worry about telling your parents? Coming out as gay holds the same fears, and likely even more so if they married/hooked up with you as a straight man.
I think the best thing to do here is take a step back, breathe and realise this is about more than just you right now. Maybe for him, the idea of being identified as gay is okay at work or in a social context but he might not be as comfortable with that when it comes to his parents.
And also, if his family react badly to him being gay, he may be worried that he panics, maybe trying to justify it like, "He was a girl..." or other such stuff that may just make you feel horrible, even if he doesn't mean it. He might just need more time to process it all so that he feels able to tell them and both defend himself and you without hurting anyone in doing so.
The point being, we don't know what's going through his head. 4 months might seem like a long time to you but you've had your whole life to process this and accept who you are. He's not had the luxury of a lifetime of processing. This is all new and even if it seems like snail's pace to you, to him it could feel like everything's happening at a million miles an hour and he's spinning out of control.
End of the day, I think the big thing you both have to do is talk to each other, just don't pressure him into anything because he could end up just rebelling against you rather than working with you, like spacerace said. The more you push; the more he may resist.