Italiandreams
From the OP's comments it looks as if the only person feeling insecure is the DP.
He is mollifying his own feelings here, using his own child to assuage his guilt.
The mother of the boy is not happy with it. The OP is not happy with it. His two children with her are sleeping in their own beds.
If the boy has expressed any insecurity or other feelings that indicate he needs special attention then that can be dealt with during daylight hours, by talking, by listening to anything the child says about the ending g of the relationship between his mum and dad, time spent together, plenty of affectionate gestures.
Quality time together this is not. This is a grown man using someone else to make his feel less guilty.
He's thumbing his nose at the child's mother and at his current partner too. And we do not know if the child has asked for this sleeping arrangement or if it has been imposed on him.
If this is a decision made by the DP on his own then he needs to stop it immediately. It's a very serious breach of everyone's rights and an indication that this man is deeply selfish. He's a man who does not honour other people's boundaries.