I recommend that you speak to an experienced family law attorney about filing a motion to modify custody, visitation, and support arrangements. Your son is a minor and he does not have the right to negotiate his own custody decisions. The guardian ad litem, and the mediator, may believe that the current difficulties between your son and your ex merit a new arrangement.
I do not practice in Washington. However, the following is for educational purposes not legal advice. You need to contact a family law attorney in your area. Some will work out payments and will seek fees from the father as well. Your son has no right to refuse to go to visitation. You have no right to prevent him from going. HOWEVER, what you outline is a senario that no court will tolerate. The court will, most likely send you to a mediator or an evaluation to determine if what is happening is in the best interest of the child and it doesn't appear to be. Your son should be able to tell the mediator or evaluator his feelings. Children are not chattel; they do not belong to either parent. An attorney will file a motion for an increase in child support and a modification of visitation. Only a court's order can modify a previous order. Do you and your son a large favor: go see an attorney and get some help.
No, 15 year old kids do not get to pick and choose what the court has ordered. But read the last two answers- the court can change support AND contact. Go see if the local Bar has referral list of free/ low cost consultations with a competent attorney. Sadly, support is tied to his income, so you'll never get all you deserve.
I am a Washington and Oregon licensed attorney. In these states, a child does not get to decide this. In some circumstances in the states I practice in, a custodial/primary parent can be held in contempt for not requiring the child to go on the visit.
The previous attorney posters have given you good information. Here are some more thoughts:
First, I'd recommend you engage with a mediator in your area to help you and your child's father talk about these difficult issues and develop some agreements and boundaries around your co-parenting. Mediation should also include your son, since he is 15, and although the law does not allow him to decide what his visitation schedule is, he should have the opportunity to express his wishes at this age.
The mediator should be one who specializes in conflict resolution and understanding family law/parenting/child development issues. This person will probably NOT be a lawyer, though that is not always the case of course. If you don't think your child's father would agree to mediate, then you investigate mediators yourself and call them to ask them their ideas about how to engage your child's father in this process.
I find that attempting mediation as a prelude to litigation is a good idea - sometimes you can avoid litigation. And if not, then everyone has good information about everyone else's perspective, and this tends to narrow the scope of litigation and help everyone get right to the core of the issues to be resolved.
Nancy Nellor Retsinas