My ADHD partner can treat my son harshly

I have been with my ADHD partner for 3 years I have 2 kids 1 from a previous relationship my older child a boy who is 8 and my daughter is 10 months. Sometimes I feel my partner does not like my son I know this is not true because my partner can be very kind to my son and takes him places and they can get along great, my son still has not come to terms with me and his dad breaking up he has had a hard time his dad who he still sees is not a good parent he is always letting him down and it's very hard to watch his dad still says stuff to him about when we were together he plays mind games with him, I try to make up for it by talking to him about it and explain things to him when he gets upset, my partner dosnt understand the emotional pain my son is going through and thinks he's cheeky and bold I do admit he can be cheeky but nothing extream my partner punishes him by putting him to his room or taking something from him and no longer consults me about it, sometimes he can be a bit harsh and when I say something he gets angry with me I try to tell him he's only a child and kids can be bold. We had a row the other day we were out in my  partners parents house having dinner while waiting on dinner I put a film on for my son to watch and my partner didn't want to watch it but I said to him leave it on for the kids when I left the room to help with dinner a few mins later my son appeared in the kitchen I asked him what was wrong he told me my partner had turned the channel so I went in and asked him to turn the movie back on and he Wudnt he said it's crap I said maybe to u but kids like it now turn it back he refused so his mother came in and told him not to be so selfish and turn it back he became very angry and verbally abused me and his mother came into the kitchen and said to my son that's all your fault you caused that his mother told him not to speak to the child or me like that, I ignored him for the evening I was upset over it, he was very nice to me and my son later that evening playing and laughing with him as if nothing had happened. Just don't no how to Handle it and I worry he will be the same with our daughter when she's older