I've been dealing with an asshole ex for more than 16 years (those are the ex-years). Our daughters are 16 and 18. My ex put me through extreme torment for many years, and he still gets a dig in whenever he can.
One quick comment regarding this comment:
Medical guidelines are not spelled out in our decree but it is the law in California. A reputable therapist will not see a child without the signature of both parents if you have joint legal custody.
My divorce was handled in CA and is still in force. Joint Legal Custody, as defined by CA means that EITHER parent has the right to authorize medical treatment or even enroll (or disenroll) the kids in school WITHOUT consulting the other. Barring specific language to the contrary, he does not have to consult you first. (I'm not a lawyer but that is what my lawyer said).
As to the spinal adjustment, chiropractors work on young children and babies all the time and given that no one wants to get sued, I suspect it has no ill efffects, though I have no clue if it has any merits (I can ask a friend whose wife is a chiro if you want). I know some believe you can cure anything with a back adjustment. I don't buy it, but some do.
As to altering custody and visitation situations, you pretty much have to prove there has been a "material change in circumstances" before a judge is going to give you any credence. As to requesting mediation, I think that's always a good idea, but only works if court ordered or both parties want it.
One spot of advice learned from painful experience, what goes on in your house, and what goes on in dad's house does not need to be the same. Rules can be different, even bedtimes can be different (though this is much harder on smaller kids) and kids are resilient. Most do okay. The most important thing you can do is to not let your frustration with your ex enter the realm of your child's awareness.
Most kids feel like they get pulled one way, then the next, and all they really want is to love you both. The less you say that is negative the better. If your son tells you things that concern you, listen, acknowledge, then CALMLY say "I'll talk to your father." Don't jump to conclusions and don't let your son see your worry. This does more harm to your child than good.
Then have a calm discussion with your ex. Again, speaking from a lot of experience, the more you ask questions and NOT attack or put him on the defensive, the better results you will have. He does have a right to establish what happens in his house and also a right to privacy. The more you respect this, the better his responses towards you will be. You have the right to expect the same. Threats usually just create more problems and a big legal bill. They rarely get you what you want.
One more caveat, as your son gets older, the more he sees you react to any "complaints" about daddy, the more he is going to begin to play you both against each other to get whatever he wants. My girls are teens and my oldest has a true gift for manipulating her dad and I against each other. She knows exactly what to say to get our "goat" up. I have found that on important matters, communicating with her dad first and/or in a three-way conference call works best because she knows that both of us have ALL the facts. I get a lot more cooperation from HIM because he knows he is being included and his view is important (or at least that is the impression I give him...it isn't always the reality).
In other words, by treating him respectfully, I've learned to get more of what I want from him for our girls.
I wish you luck and I realize this is not likely the only problem you've had with your ex, but from the little information you give, I think you over-reacted. I'm not defending him, I am just saying that it seems that he was trying to do right by your son (by taking him to a practioner, even if not a traditional one) and that was within his rights as a dad.
Take care.