Talk to him, and tell him what he is doing. I believe you had mentioned he had pretty intense anxiety so setting these boundaries may cause some drama.
This level of neediness can be quite draining.

Restrict online times to an hour in the morning, and Then however many hours your comfortable with in the evening.
Explain that just because your online it doesn't your always available to talk. If you need your own time to do stuff say so.
Also provide yourself a half hour window when you say you will contact him. But ensure that you do, or explain why you can't, or your going to risk triggering him.
Can I ask what happens if you don't go online when he is expecting you?

It simply isn't practical to be online constantly and if he is doing that what is he doing in order to live and be productive at his end.

If he isn't sleeping he isn't taking care of his mental health. This is something you could gently impress on him.

I suspect this level of anxiety and neediness wouldn't be limited to online. So it's something you need to get a handle on if he is planning on coming over.

What does he do when you aren't available, or need to. Change plans?
Just trying to get an idea of the behaviour your contending with.

( I want to add I think you might encounter a lot of these kinds of issues. With this young man.)