Responses
Saying "no" to him? "No" on one thing nobody ever refuses and breaking his coping mechanism? Making him feel like he's being abandoned again? I don't think so... Sex, good aftercare and a promise to never leave him. 3P relationship.
At the very least a conversation before starting.
Clearly he is using sex as a coping mechanism. Because he had abandonment issues and don't trust these two.
But it would probably serve both of the tops better if they didn't succumb to our needy bottom.