Agree with all the comments here, but just a thought about your dp... If he is a decent man, it seems like he's fallng into a comfortable role for him, that of obedient child, & relying on his parents to be the benign authority who he can just go along with. However he has to break out if this, as his parents aren't the heads of your 'almost-family', you and he are! I can understand that it's very easy & feels 'right' to fall in with his parents wishes, as that's probably been his default position growing up, but now he is grown up, & has more roles than just being his parents child. He has to think about how to be a good partner & a good father as well as a good son now.
I am sure he may be shocked at some of the posts here, which clearly say that from mmsnetters point of view, his parents motivations are highly suspect. I hope these opinions don't make him defensive & unableto get past wanting to defend his parents.
Giving his parents a Ginormous benefit of the doubt, they are thinking about his interests only, not the couples, or the family. It may be that they are doing this unintentionally, perhaps assuming that what's right for him is therefore what's right for you & baby, or maliciously trying to cut you out (that would be my opinon btw but trying to take a generous stance here!). The issue is here, not why his parents are offering this, but that the substance of what they are offering works for him as their son, but not as a partner or father. If he truly wants this relationship to work, & to become a daddy, then he needs to break out of deferring to his parents wishes & take on that grown up role himself.
I have a 5 month old & looking back over the last few months, if my dp had not been living with us, he would have a very different relationship with his son. I cannot over emphasize how important these few months are for bonding.... It's the little moments that love springs from, like laughing at the milk drunk baby who is comatose on your chest, or working out how to change him/ burp him together, or him making up a dilly song whilst pacing the floor with screaming baby at 4am.... You can't get these moments through a quick visit once a week... You would end up being the persn who instructs him on how the baby needs to be held/ fed / cuddled etc rather than him being fully invested...
Maybe (benefit of doubt time again!) his parents had a very traditional aportioning of role where the father didn't do any childcare, & so that's why they think it doesn't matter if he misses out on these things, however, does he really want that for his own model of fatherhood?
Good luck & I hope you don't end up having a horrible argument this evening. I guess my advice for you to get across to him is:
He needs to move away from just being the child to also the new role of partner & father (obviously he ll always be their son, but he can't be the son at the expense of other roles, he has to manage all his responsibilities).
Just because he thinks his parents are offering this with the best of motives, does NOT mean it's the right thing to do. His parents may need help in moving from treating him like their child to understanding he is grown up, & that is something he has to show them.
Assuming he can catch up on the missing 6 months is a wrong assumption as without those 6 months it's doubtful whether he'd have a future living with you & his baby.
(sorry that was really long, got a bit involved in your plight!)