Hi and welcome
I'm sorry to hear of your situation. It's difficult to accept a break-up when there are particular reasons, and perhaps less so when there is no apparent reason except something within the mind.
Mental health is unique to us all (despite similarities to other cases), so I guess you may have more insight as to where his thoughts were or are. What I can say is that at times we might arrive to a point where we feel the need to change something suddenly, with or without knowing why ourselves. For myself I need time alone and also time with my partner and if possible the few that I do care to be around, but my partner doesn't understand the times I want to be by myself.
I wonder how your relationship is with him now. Do you talk and keep in touch? Does he mention other things going on in his life? Knowing where you are at the moment might help to advise a touch better.
Here should I hope be a place you can share as much or little detail as you like and anonymously.
Depending how you both are, a simple "I hope you are ok" or "I'm here if you need to talk" without pressure. Talk about yourselves as a couple may not be the right way for now, but he will understand the confidence that is there and hopefully trust in you.
For yourself, it is difficult to lose somebody like this. But as per your post, if you are to move on or not, you need to come to terms with where you lie with his continuation of all - this much is evident from your post. You will not feel better until you understand more about where your loss of him or his well-being lies. You are in the area of analyzing all I guess, so I hope sharing might help you here.
Welcome again, you will find support in your endeavors here