Thank you all for the responses, and sorry for my lateness in returning to this thread. The child and his parents are located in Colorado.
I thought this was implicit, but yes, the father's interference is detrimental to the child. He objects to necessary medical care that would improve the child's quality of life - necessary adjustments to medication, nutritional changes, and programs that would improve his functioning. The kid is miserable, my mother is trying to find different avenues of treatment for him, and the father objects to everything she attempts on principle. She has attempted to discuss different avenues of medical care with him, in case he truly does have an issue with the way he's being treated, but he ignores these conversations. I understand that withholding medical care information from the father is not appropriate, but she started doing this recently because of his interfering with care - and it hasn't worked anyway, because he re-routes and gets the information from the child's care managers. There is no great, grand conspiracy against this man. We do not have any kind of sinister ulterior motive. My mother is simply trying to seek necessary medical care for the child without him objecting to it, because he needs medical care and the father will not allow it.
I should also mention that they live in a smaller city, and the medical care there is subpar, so it is not that surprising that doctors are not advocating for her to do things to change her situation. Several doctors in their town have flat-out told my mother that they don't know how to treat the child, and have either sent her away with nothing or given her a referral to another office that eventually does the same thing. I understand how that might sound unbelievable, but that's really the way it is over there. Recently, she's been unable to seek medical care for the child both in the city and beyond it because of the father's interference.
Somebody in the thread already said this, but my mother takes care of four children under the age of 10 on a regular basis, and she works. She does not have time to reach out in this kind of manner. I understand that I am removed from a legal perspective, but I have always participated in care of the child and my other siblings (I moved very recently). I do not feel that I'm out of place reaching out for help on her behalf.
My mother's concern is that, if she were to successfully revoke medical custody from the father, she would either lose his court-ordered visitation with the child - which she uses on weekends to work and do other things - or that he would counter and try to take the child away entirely. She understands that a revocation of medical decision-making is necessary, however, and my question was largely asking if there are legal resources or programs for special needs children that could help her pursue that avenue - because neither her nor I have the money to hire an attorney. We are both saving up to hire one, but that could take months. I have told her to speak with the city's legal services in the area, so that's a possibility, but we're worried they won't want to touch it because of the complexities involved.