Of course the insurance counts. Your ex is obligated to provide insurance for your son, and he is doing so. The fact that its coming through his new wife's employer is irrelevant. Your son is insured, and therefore the ex is meeting his obligation.
Does your divorce decree clearly state that your ex must provide insurance where the current pediatrician is "in-network"? If not, then there isn't anything you can do about it. He's providing insurance, and that's his obligation.
Even if your ex had continued with the previous policy, there's no guarantee that the current pediatrician would have continued to be "in-network" with that insurance company. Provided networks do change.
Through my job, I've seen hundreds of divorce decrees. (At least, the portion that pertains to providing health insurance.) I don't know what your specific decree states, but compared to the common provisions in the ones that I have seen, your ex is likely fulfilling his obligation.
If you have reason to believe that there are unusual and/or non-standard provisions in your divorce decree that would make this a different situation, then you should contact your attorney to get his/her input. But if you have the standard language about health insurance that tends to go in most divorce decrees, there isn't anything you can do.
P.S. I understand that it upsets you that your son may have to change the doctor who has seen him since birth. My kids saw the same pediatrician from birth to age 5, and then we had to change because I got a new job. It happens to a lot of people, for varying reasons (not just divorce/custody issues). But he will get through it okay, and you will get used to taking him to a new doctor. If you absolutely can't accept changing, see if you can continue seeing the old doctor "out of network." However, you'll have to be prepared to pay more out of pocket for the out of network costs.