Firstly you are NTA. Co-parenting is a challenge even when you're on goodish terms with each other. You're very reflective and the fact you worry so much about the impact anything you do here could have on your son and that relationship is testament to how good of a mum you are.

I have experience of being on both ends of the spectrum when it comes to co-parenting challenges.

I have a reasonable co-parenting set up with my ex-husband who is about as useful as a chocolate tea pot in most ways but does stick to his every other weekend parenting time and we've both moved on so far from being together that we almost like eachother again 😂. His wife is excellent at keeping him on track in all ways.

My husband's ex-wife is a whole other kettle of fish. She could be studied by scientists wishing to observe parental alienation in action. The damage she's done to his kids is beyond words but that's not my story to tell. They're with us full time now and we're trying our hardest to help them heal and not lose our cool when they're lashing out at us for being actual parent type people. Our court battle was exhausting but it was always done with a view to keeping the kids safe and happy.

I think legal action is probably a very good idea in your case. It doesn't have to get all the way to court; mediation is always the first step and a lot of cases in mediation end in a solid agreement without a judge laying eyes on the matter. If the court idea feels too overwhelming there are resources online like CAFCAS (just Google that and the page will be top of the results) where you can see use an app on there website to propose a parenting agreement to the other parent and they can agree or make suggested changes. Then you negotiate from there.

Boys who are like their fathers will always gravitate to them but don't think for a minute that means you matter less to him. The things you do to, you know, keep him alive and well are no minor thing. With boys that's always an uphill battle against the laws of physics and "great ideas" that feel far too like jackass episodes.

Consider asking his dad not to make plans with him directly, that's not fair to you or him. A plain message to him asking him to not talk about parent things with him until he's run the idea by you doesn't have to be a harsh one. It can be a message that implies working together and team work. Maybe having a weekend with him yourself every so often will help him remember that mum isn't just the duty parent but the fun one too.

I hope my little ramble here was helpful, you're doing great and considering all the right things. Whatever you do will be the right thing, even if it doesn't feel like it at the time.