Cancel any plan to move in together.
I'd consider cancelling the engagement as well, because your DP is horribly demanding - telling your boys & you how to behave in your own home, & believing that he alone is the arbiter of what the blended family dynamic should be.
FFS this is going to become horrible if you cohabit.
DP is already throwing his weight around & instructing you & your boys to give his DS preferential treatment.
It's a power play. Your DP thinks he is the dominant party in your relationship, & that your family ought to do as he tells them.
You reassuring your boys that they don't have to is NOT a solution. Try that one if you are daft enough to cohabit - DP will entrench, escalate, & you will be constantly on eggshells looking to mediate & fending off DP's unreasonableness.
That is no atmosphere to raise DC in.
i also over heard a conversation his son and mine had this weekend where his son was lecturing mine about how he shud give him all his attention when hes in our house. they get on great but i did find this very odd..i obviously made it clear to them both my son was not expected to do this
Because DP has trained his son to believe that this is a reasonable expectation, & that he is entitled to it.
Please don't do this to your boys - or yourself.
Prioritise them, & keep the relationship, if you still want it, in separate houses.