Disclaimer: I am not licensed to practice law in Washington. This answer is provided as a public service and as a general response to a general question, it is not meant, and should not be relied upon as specific legal advice, nor does it create an attorney-client relationship.
The dilemma: Do you want what you want, or what is best for your child?
The basic problem you appear to be overlooking is it better for your child to stay with a "friend" instead of with his father? You have made a commitment to attend an out of state training program, even though you have parental responsibilities. Your son has a father that lives in the state of Washington where your present residence is. Unless your ex is an unfit parent, or poses some sort of threat to the safety of your son, it is likely a court would order the custody transferred.
Consider working out a temporary change of custody pending your return from training. The approach your ex is taking, requesting a mediation to change the parenting plan is not unreasonable. When you think about it carefully, you may realize that it is more important to think about what is best for your son, not what is convenient for you.
I suggest you meet with a local family law attorney to help you draft a stipulation that would protect your future custodial rights and the best interests of your son.
Good luck to you.... and your son.
A lot of people talk about "joint custody" and it's hard to tell exactly what they mean. It's not totally clear from your question if you have a shared residential schedule, which is really what's important. When you mention a mid-week parenting plan, do you mean you exchange the child once and you each get the child for a full week? Or, do you mean that the ex gets a visit in the middle of the week in addition to his weekend visit. I'm thinking the latter because you mentioned a weekend schedule as well. That assumption is what I'm basing this answer on.
If the child lives with you except for ex's visitation weekends and a few mid-week evenings, then changing the parenting plan would be hard for your ex. He would have to show a substantial change in circumstances for you or the child. Obviously your absence at training may qualify. He would also have to show that the benefit to the child of changing the parenting plan outweighs the harm of making such a change. In addition, he would have to show that either: 1) you two are in agreement with the modification, 2) that living with you is detrimental to the child's development and welfare, or 3) that you've both not been following the parenting plan anyway. As you can see, it's an uphill battle by design. But don't assume that you are in the driver's seat here.
Normally a child would be staying with the non-custodial parent under your circumstances. You need to focus on why that shouldn't be. How long is the training? The longer the training period is, the stronger his case becomes. You need to be able to show how keeping the child where he/she currently lives matters. You've mentioned the continuity with school. That's an excellent point. I imagine keeping the child with his/her friends, keeping child around familiar adults and health care providers are also important.
If your training period is more than 4-6 weeks, I suspect a court would temporarily modify the parenting plan as long as the child's schooling wasn't affected. Courts don't like uprooting children from their schools or making them change school shortly after the school year starts. Of course, some of that depends on who is assigned to your case. I can think of some on the bench who would temporarily modify and others who wouldn't.
As for the last sentence in your question, the idea of seeking "full custody" so you can make decisions on your own is a non-starter. In Washington (and it's in your parenting plan in the Decision-making section), joint decision-making is required unless you meet certain criteria. This case would not meet those criteria from what you describe. Your ex trying to get the child to live with him not rise to the level needed to change to sole decision-making. Any parent has a right to ask to change the parenting plan (and the fact that he's using dispute resolution to do is actually a good sign).
As a last word, I didn't care for the tone of the other answer. Sometimes friends are far better parenting substitutes than the other biological parent. Some parents are ideal in 2 or 3 day increments. I don't know you or the dad. Maybe the dad would be just fine. However, I don't think you are simply being selfish as I've had clients in your situation. But, that said, you really need to watch your tone if this ends up in court. It's easy to read your comments as selfish and controlling. The other attorney is correct in that you need to be very clear that this is about the child and not about your plans getting messed up. I hope this works out for everyone involved.
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