@Fortheloveofaboy I had this too. I remember going on holiday with him in the early years and spending neatly an entire day once, trying to find a phone so he could call his family. I hadn't occurred to me to call mine because they knew we were away on holiday.
I once came back after spending Christmas with my family, to find his eldest sister had rearranged my lounge furniture in my absence. Because it was 'his' house and they were so used to controlling him it didn't occur to them this was grotesque. But it really magnified how irrelevant I was to them.
We were never involved in family plans for events. His parents and sisters decided everything and we'd just be told this is happening on x date in this way, and I would be expected to just go along with it, and judged if I didn't.
Birthdays were so shit. DH was obligated to see them on his no matter what. We could never spend the day as a couple. When that extended to me being expected to also accommodate them on mine I put my foot down. And I made damn sure, when we had DC, that our children's actual birthdays were for us alone, making time for his family on a weekend before or after.
I understand more now why my DH was incapable of dealing with it, but I don't think I've ever fully forgiven him. He was quite pathetic and wet. It really made me deeply unhappy and affected my self-esteem. What DH really wanted was for me to do what he did: suck it up, go with the flow, basically do what I was told. It was suffocating.
Interesting - he can't remember a lot of these details. I think because being managed was so normal to him that none of this was stand out shocking. Either that or he feels ashamed or is still in denial.
Also, if it's any reassurance, things dramatically improved with the absence of his parents. I now love my SIL and her DH. Before, I think his parent's views had strong influence there too.
Honestly, my best advice is establish some hard boundaries and be inflexible. Tell your DH what they are and don't budge. Your DH will stand by and watch you bend and struggle to fit in until you break. Because that takes the pressure off him.
Also ... move house further away. He'll stand a better chance of growing up if his parents aren't around to constantly infantilise him.