The Government just passed about 400 pages of new law regarding guardianship and stuff last year. In my Grandparents Raising Grandchildren's group we have an attorney from OKC who specializes in this field and she is on many committees for making these laws. There are more options now and one of them is a legal paper that gives you the right to enroll someone in school, seek medical treatment, etc...but does not take away parental rights. It is a brand new law. I can't remember what it's called but it's like having a "permission to treat" letter from the parents while you take their child on vacation with you but it is signed by a judge and legally binding so a hospital or physician can trust it more.
If you want to take this child away from his parents then you have to file paperwork in the court system and show cause why the court should give the child to you and not leave them with the parents. You are responsible for all materials presented. You will be responsible for getting testimony of credible witnesses to try and prove your statements, you will have to pay the attorney fess for your attorney and if any professional witnesses have to be paid you'll have to pay them. You would be accusing them of being unfit parents and you have to prove it to the court. They only have to show up and say it's not true. You have to PROVE your case beyond a shadow of a doubt.
We had 4 Child Welfare Workers, 6 witnesses of how my daughter interacted with her children and how often she came to see them at my house, etc.... We had monthly calendars to show what times she had dropped the kids with us and when we saw her again, when she picked the kids up, if she took them for any amount of time, what activities we had them involved in, etc...that was our proof of what we did on a daily basis. But the professionals were the ones who won the case for us. They have credibility in court.
I hope you and his parents can come to an agreement and just do some simple court document that will allow you to help and not be a huge long drawn out proceedure.