What should we do?

We currently have placement of my stepson around 30% of the year. The only reason as to why we don't have more is because it's a 45 minute drive to his school. We also went to court when he was younger and the GAL thought at the time the drive would be too long for him. This is when he was 4. Now he is 9 and we've been contemplating on going back to court. Anyways he is with us during the summer week on/week off and the the school year holidays and weekends. During the summer I am home with him as his Dad works. We'll when he's here I'm home with him I take care of him. Make sure he's fed, clothed, happy, and take him and his siblings on trips. Anyways we don't get along with his mom due to past issues. This has created huge issues with my stepson. He lies to his Mom about things. We asked him why and he's told us that he is scared of her. We've told her about this and she doesn't think it's true. We've witnessed it happen numerous times. He'll also tell us how he gets bad anxiety with her and her parents. Anyways he ended up in a boot for hurting his foot. It wasn't broken or anything. It was more for comfort. Well we were having a birthday party for him and his sibling that weekend he had the boot. We told him he was unable to take his boot off to go into the bounce house and unfortunately had to stay out. Well when we got home he immediately took his boot off and went in the bounce house. I told him that his mom is going to end up finding out and that he will be in trouble so he needs to listen to her and keep his boot on. He straight up said to my face "she won't find out!". That was the last time I ever said anything about his boot. The next day his Dad told him he's either going to leave the boot on the rest of the day or leave it off because he's not going to have him running around with it on. I had no say in this conversation nor did I want to because it felt like it wasn't my say. This last weekend we went to go pick him up and his mom told me that he said my mom, sister, and I blocked him in a corner made him take his boot off and get on the bounce house. My family wasn't even here and they even showed up late to the party. I was so devastated and broken that he would say anything like that about my family and I. My family has always been there for him(his mom doesn't like it). They have never treated him unfair. They've always bought him gifts for Christmas and Birthdays and offer to take him places. I'm now at the point in life where I am exhausted. My husband and I argue about it. It affects my other children and I no longer know what to do. What do you think?