M.6.
I'm really sorry you are going through this - I agree with you, she is definitely overstepping her boundaries. However, often this isn't as much on purpose, as it is on accident (which doesn't make it hurt less). It sounds like dad and step-mom really stepped to the plate when your job required so much of you, so it would seem kind of petty now that you don't "need" them, to start stamping your foot around going "mine, mine." I'm totally not saying that is what you are doing at all, but it might seem like that to them if you are not careful.
Kids are funny creatures and creatures of habit so if he has been at dad's more for the last year, that is where he is going to feel most comfortable. However, the more time he spends at your house, the more likely that is going to even up a bit. Sometimes, one parent's house is just more fun, though (like if he has step siblings to play with at dad's but not at yours). Kids love fun and they will push for fun when they can. Try not to hold that against son or dad.
If you have more time, I would start with volunteering at his school. It is a great way to get connected with your child in a way that is meaningful AND if give you the opportunity to see what projects he is working on so you can offer to help and also have some input on certain projects.
Remember, your son isn't on social media and could care less about the pictures and the postings :)
Good luck!