What is it that you are considering mediating? You didn't really say. If someone is suggesting mediation, it means there is an issue you are trying to amicably resolve, and reading your post it's not clear to me what that issue is.
You began by saying that you had a verbal agreement for your son to stay with his dad for 3 years, but now it's been 4. Then later you mentioned that Dad has petitioned to stop paying child support (logical, since the child lives with him, and I'm surprised he's been paying all of these last 4 years! unless it was repayment only for back support/arrears owed?). Then you talk about an argument you've had with the step-mom, but don't say what the argument was...only how she went about spreading the discord that resulted.
What aren't you saying here? What is the mediation supposed to resolve? Hurt feelings? That's not normally what a mediation is about. If he has the child living with him, and you don't plan to try to force him to move back with you, what is the actual problem?
It would be helpful to know if you want to know whether mediation could be beneficial.
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After your added information, I would just suggest you talk to his dad directly, and do not go through the step mom at all anymore. Maybe he doesn't want to talk to you. Maybe it's just her not wanting you to talk to him. Who knows. But if he doesn't accept your calls, then forget him, too, and talk to your son only. You said you do not have any formal custody papers, but you *do* have legal court orders regarding support, correct? Operating under the assumption that there is no requirement for either of you to contribute to college expenses, then I don't see the necessity to discuss schools with the dad. Sure, it would be logical and nice, and make things easier, and be smoother and simpler for your son... but it isn't *necessary*. What will be necessary is for your son to communicate with you. He may not realize it now, he's only 16 after all. But when it comes time to PAY for his schooling, and submit his FAFSA information, he will HAVE to have information from you. And if he continues his education into college, he will need financial means or loans, and I'm sure he will be counting on you for those things, at least some help anyway, also. He may not understand the ins and outs of out-of-state tuition and the like, yet, either, but he will. And you can help him in your discussions WITH HIM. His Dad may be having these conversations with him also, and if so, GREAT! But you don't have to talk with dad. Just your son. Ultimately, what he decides to do after high school will be up to HIM. Not you and his dad.
(Unless, I have misunderstood somehow, and you were referring to schools for his junior and/or senior year of high school?)
I'm not sure a mediation is warranted just to discuss things that you haven't yet discussed, and that you (parents) don't have the final say in anyway. Once he turns 18 (17 in some states) he's a legal adult and HE decides what he's doing, if he's going to school, what school, if he takes out loans, how he's going to pay for things. Just talk to him. But understand that when you talk to him, he's still 16, and may feel like it's a long way away and not a major priority *right this minute*, or that he thinks he's got it under control. Don't treat him like a child, just let him know that it's getting closer to the time he has major decisions to make about his life after high school, and that if he intends to pursue higher education, you'd like to stay in the loop with him so you can know how you can help, either with questions, finances, or information he needs to file forms/paperwork. Stay open and interested and try not to sound too *motherly*. That's my advice generally with teens this age. They think they know everything and have it all under control and don't need Mom and Dad. And if they do need advice, they often tend to head to dads first, b/c dad's tend to not treat them like their little boy babies like moms have a tendency to do (unconsciously). I have an 18 year old. This applies even to me. So please don't think I'm accusing you of anything. It's just how teen boys think/behave when trying to separate themselves from parents and growing into men.