Being a step-mom has changed our relationship

I’ve been with my SO for a little over a year.. we got engaged back in may. We had a wedding planned in October but put it off because we started having so many things hit us at once it just didn’t seem like the right time.

He has 2 kids ages 8 boy & 11 girl
I have 3 girls. A few months ago his oldest started telling her mom she didn’t want to come over anymore… she started her period, started school, I figured it was typical girl things and her probably wanting to just spend time with her friends on the weekends instead of coming to her dads. He still made her come because it’s the only time he gets with them. Eventually she started telling her mom she was scared of him… (he’s never spanked them, or had to disapline them while they were at our house) it came as a shock because there both super good kids, & always acted so respectful and genuinely happy when we had them! He decided to give his daughter space thinking she would come around… his son the youngest still came over… we were told his grandma and mom were telling them to say things like that…
he continued coming for a few months then when we were supposed to get him thanksgiving he freaks out… says my girls ages 3,6,& 9 bully him… they’ve always gotten along so well besides the typical kid argument because the age difference with some. But again, never had to get onto them… well his dad didn’t force him to come after the mom making a scene in the parking lot saying she was going to kick everyone’s a$$ & his son was traumatized by this point… he wasn’t about to yank him out of the car and make him come… so we tried again on his next weekend… this time he freaks says his dad threw a knife at him… then said he “said” he was going to throw a knife at him”… the stories kept changing… his oldest daughter had text messages on her phone saying she “can’t stand these brats, & her “dads new bitch won’t let her have any fun” these were sent from her… to her mom… and her mom replied with “lol” as if that’s how she and her 11 year old talk… I know my kids are far from perfect… but they’ve been taught not to talk like & his daughters always been so close to me everytime she comes over it was just a shock. when there dad was working I would watch them, take them horse riding, go to our property in Oklahoma and kayak, fish, we always tried to have fun stuff planned on the weekends they came…
now neither of the kids want to come over… He’s clearly upset about it but feels forcing them to come is going to make them not like him even more… he says this isn’t the first time that there mom has got stuff like this in there head and made cps cases against him… which he passed because they said she didn’t have a case and no proof and that he was a good dad etc… he’s planning on calling his attorney but saving $…
I know this is super hard for him as he’s cried to me about it a few times but I feel like it’s completely changed him as a person. Which I can’t imagine my kids saying they don’t want to see me again… I try to put myself in his shoes and be understanding & ask what I can do to help. I feel like it’s starting to effect me and my girls because he’s not happy and takes it out on me… not abuse just is short tempered, doesn’t want to talk to me much, he stays on tik tok all day but as soon as I go to have a conversation I’m accused of “trying to start a argument” and he’s all of a sudden tired. I just want my girls to see a happy healthy relationship, and a happy mom because my past relationships were not the best. I’ve tried to be patient but im at a loss of what to do & what’s right at this point. He doesn’t have much family, he doesn’t talk to his parents, his kids want nothing to do with him… he has close family but they live hours away. I feel like me & my girls & my family are always so supportive and do everything we can to make him happy but sometimes it doesn’t seem appreciated or like it would bother him if something happened to our relationship. I knew being a soon to be step mom would be different but I’m just at a loss with how it’s changed our relationship so much and I don’t want to be the one to give up but I don’t want to fail my girls by putting them in this situation where they see people being unhappy. Any advice!?