OP I had a nervous breakdown at the end of July (mostly due to care of elderly mother but with hindsight, definitely other things too).
I was seeing someone at the time and he tried to help a lot, but I left him not long after (I don't normally date or have relationships or whatever).
I recovered very very slowly (it felt to me) and no one could see a difference in me till about a week ago. In November I felt I was teetering on the edge of another one but that was very much due to particular circumstances.
Controversial opinions incoming....
I did wonder if I'd have recovered faster without the boyfriend. My situation is unusual but I've never had anyone to help out before.
Is it possible your boyfriend would rather be left alone to recover?
As for whether he'll come back, I don't know, but my experience of mental health issues is that I become a much quieter version of myself every time they hit hard. If he recovers and becomes a very different person, you might want to move on anyway.
We all have limits of what we can cope with.
What is your relationship like at the moment? If he's in a complete daze, he might not care much if you leave. He might want to end the relationship himself but cba.
Or is he leaning heavily on you for stuff? Are you looking after him or do you just feel the "original him" is absent? You can leave for any reason you want.