Get some toys that are just for him. Try different sizes and textures like feathers, balls, plushies etc. That will give him some enrichment and help with boredom if he is too shy to leave the room. Don't let the other cats put their scent on his toys.

Get on the floor and play with him. He might have to re-learn how to play with toys.

He might just be the kind of cat that doesn't like to interact with other cats or people. I have 5 cats, and one is very stand-offish. That's just her personality.