The situation strikes me as similar to many other anxieties. A person feels anxious and asks for reassurance. If the reassurance is given, the person never learns to judge the situation for her/himself and never learns to tolerate those feelings of anxiety.

A slightly different approach would be to accept the question but decline to answer. So when he says "Are we okay?" a response could be "What do you think?".

Presumably he will say "I don't know, that's why I am asking."

Then one could say "Is there evidence that we are not okay?"

The idea is that he then is expected to produce evidence or realize that he doesn't have any. If he produces "evidence", that can be discussed and he can learn what constitutes actual information about the relationship. If he has no evidence, you can simply say "Okay, let me know if you ever have any actual evidence."--without giving him reassurance.

Over time, he can learn to look for evidence one way or the other, rather than simply turning to you for reassurance.