I think you have to give it time. I don't get responses to I miss you, either.
Aside from my initial panic phase which lasted about 4 weeks from him withdrawing and shutting down, I've gotten myself into a good, strong place now. It's sort of like acceptance. We have always been close and instinctually knew what to say when. I can be in the middle of writing a rant and just before I hit send, a message will come from him and I'll delete, delete, delete. So I'm hoping that me now being in this calm, accepting space, that that will find its way to him. Less pressure from me = him being more open.
Everybody is different, but if you could try and get a good hold of yourself and lose the need (for now) to require validation from him then that would probably be helpful. You're not being emotional and he's not getting a vibe that you need something from him. He has nothing to give at this point, he can't even give it to himself.
I really do know how hard it is, and I hope this helps. Time, time, time.