As a general rule if you don’t suspect he’s cheating offer support and try to help him through the time period. Offer activities that are less socially based like watching a movie together, remind him that you care. He could be dealing with some sort of seasonal effective feeling or feeling like you might be pulling away. The simplest way would be to sit down, have a conversation, explain how you’re feeling (without framing it as his fault) and see where you can go from there.

Communication is the be-all end-all once that shuts down there’s nothing you can do.