I'm sorry this is happening to you. I had at one time, a relationship for 13 years that ended. You don't know yet what is happening, but I do know how awful it is when things change.
Support him until you can get more information. You'll have to be patient until then. If you think he may be suicidal then get help for him, but if it's him separating himself from you, you can only wait and see what he does.
Stay close to family and friends, know that things change and can be very hurtful and discouraging, but things always change it never stays the same and most of the time for the better. If he decides to end the relationship, please protect yourself from the aftereffects. Stay close to your family, but most of all see a councilor so you can deal with it and get through this awful time. Please be safe and hang in there. Let us know what happens.
(((((hugs)))))
Janie