I discovered a few weeks ago that xh has been using an unsuitable car seat for dc. Dc outgrew this car seat over 18 months ago. He cant even sit back fully, has to sit at an angle, and the straps do not even go over his shoulders, they hang under his arms. I am disgusted that he has been travelling on dangerous roads like this for so long. After asking him 4 times to get a new one he has agreed. He bought a second hand one. I have previously explained this is not recommended and not safe. When I reminded him of this he changed his story and said actually his sibling had given it to him. Siblings child is only slightly older and barely taller than our dc. So he wouldn't be moving on to the next group seat yet. Obviously he is lying so he won't have to spend 35 quid on his children.
Aibu to insist he buys a new seat. He's not stuck for money, regular holidays and all designer clothes for himself.