was your partner married to his x or does he have parental responsibility?
when i first got together with my fiance he was very much like that it was still a case of she asked him to jump he'd ask how high, she took advantage of him big time in the fact the kids lived with her so she thought she held the key to everything, when the divorce was going through and it was all in black and white he has joint custody and joint residency etc he wouldnt let her run his life anymore.
if your partner doesnt have parental responsibility then i suggest he applies to the courts for it, then when he gets it he wont feel the need to be extra nice and do what he's doing