How can I claim my child on my insurance policy if he’s already claimed on his dad’s?

I hope I don’t sound wrong for this. Me and my son’s dad broke up about 2 years ago and we’ve been co-parenting without an order, things have been going smoothly although my son’s dad only sees his once a day every other weekend. He doesn’t take care of anything other than spending a couple of hours with him. I’m the one doing everything else, like getting him to all of his dental and medical appointments.. I had no clue I didn’t have my child on my insurance policy. I have had iehp all my life and I don’t remember the process when i signed my child up for it all I know is he’s had the yellow IEHP card for awhile now. We’ve been able to use that for his appointments until his first need of dental surgery has arrived just yesterday.. I was told he needed the state insurance card so I went down to social services to get one and they said he’s not claimed on my insurance but he’s claimed on his dad’s. I went straight to my child’s IEHP member account to prove he’s under my address and such, because I have no clue how else to go about these things, and there was a change of address and phone number that doesn’t even match what I know of his dad even having, so I believe his dad possibly moved and changed his phone number which is none of my business but knowing my child’s information is changed to something else is my business. I’m not even able to get him his dental appointment now. I was able to change all his information back to what it’s supposed to be, but he’s still claimed under his dad. Can someone walk me through this ?