From what you said, it seems to me that something changed in his life. Most likely traumatic (maybe not to that extreme), or emotionally confusing.

I've been in that situation, and it's delicate. This might not be the case keep in mind, but it could be. I suggest you try to talk to him more, probe his mind but not in a way that's intrusive or 'nosey.' And if he responds negatively, back off and be patient.

Good luck, I hope you find him.