Perhaps at this reunion, he met people who he thought to be more successful, more interesting or having a more fun lifestyle. So he's just going through a phase of self doubt and depreciation.
On the other hand, he may have rekindled or started a romance with someone he knew previously from school. You really need to speak to him, as he is the only person who knows what's going on in his head.
As you have a child, I doubt any court would expect you to vacate your home and I would expect he would have to continue paying the mortgage and pay towards your son's maintenance.
You really need to seek some legal advice from a solicitor specialising in family law. I think most firms will alow up to an hour of free advice. If not then contact your local Citizen's Advice Bureau.