Ex has child sleeping in a walk-in closet.

Ex bought a house. His girlfriend has 2 kids and they bought a 4 bedroom house. He has put our son in a walk-in closet in his master though.

From what I can see this isn't illegal and CAS really only looks to the fact that the child has a place to sleep. Especially since it's not 50/50 nor the primary residence. But is that factual? Or is there anything I can/should do?

It sounds as though there's no window or light in his "room" and since his first visit there, he is now afraid of the dark so it seems that using this room is already detrimental to his well being.