My step son wants to live with our family and no longer with his mother, he has expressed this for a few years.
His mother punishes him by not allowing us to see him for months even years at a time and this upsets him very much.
He has been getting into trouble at school having been suspended 3 times this year already and also his behavior problems at home too, however we never have any problems at our place he just likes to sit and talk to us and play with his half siblings.
We are having him this weekend and he has said he doesn't want to go home, at this point i am happy to let him stay but his mother is going to crack it big time.
He is 13 turning 14 in 2 months is he old enough to make this decision on his own and have our full support?

You haven't seen his dark side. That's not to suggest anything bad about him, just that teenagers with behavior problems tend to hide them well. For a while.

If you want him to live with you, I suggest that you have him ask his mom to go to counseling with him for 3-6 months. They can either work out their issues or come to terms about him leaving. By addressing their relationship, it can become clear to her that he is better off with his dad and you. And, through the same process, he can gain tools to work out his behavioral problems before moving in with you and afterward.

As backup, if you do a Google search on Emancipation of Minors, you can learn and have him learn the law, then decide together if it is worth it for him to declare himself emancipated. If so, back him up all the way until he is free to decide where he lives.