Thank you for your suggestion, imd63!

Well, another update: he broke up with me.
He wrote a pretty cold email (yep an e-mail, he's probably scared he can't go through it in live):

He wrote that he had lots of stuff going on, stress, that the last day of our little vacation was "horrible" (the fight he started because he was upset about some bad prof. news. He still has no job, or even benefits until he has a job).
He said he didn't need that stress and that he's "not really in any position to be in a relationship right now". He said he wanted to get through rehab by himself.
Ending with "We can stay in touch but I don't want more, sorry".

I want to answer but it's been 4 days and I have no clue what to answer.

Does he really want this break-up? (he tried to break-up several times in person and over skype previously because of "bas timing" and "fighting" but he back-tracked every time)

Do I want it?

I know that we can't go on the way we are now. That's the only thing I'm sure about.

If he doesn't really want the break-up (big IF), going much slower, with more space (separate flats) and letting him go through rehab without interfering, would be best.

What should I do? Answer the email? Say what?
When should I answer it?

He's still in my facebook contacts. Nobody is contacting the other one.
He goes on facebook several times a day and has added all his childhood friends.
I go on posting on my wall as usual. He must be wondering whether I got his break-up email.
He's still using as profile pic, a pic I took of him.

I don't want to sound desperate, I'm pretty exhausted myself but if there's a chance, I'd like to try the new approach.

I told him 3 weeks ago very clearly that friendship was out of the question! Why does he write now "we can stay in touch"? To keep me around just in case? Does he think I'm desperate and is trying to be nice? or is he unsure?