S.H.
You should be able to get him a medical card. Do you have something 'official' (notarized or thru the court) to show that you are a gaurdian? I'd call medical assistance and ask about applying. Also, you could call CYF to ask for advice. They deal with these issues all of the time and could point you in the right direction. If you think this will be a long term thing, you might want to get CYF involved. If you don't have a placement thru the court, the parent can come back at anytime and take gaurdianship back. I don't know what the situation is, but you might want to ask CYF about that. You can call and not give your name and get some info. Good luck!
Editing b/c I see you are in Butler Co? They have their own child welfare agency and I'm not sure what the name is. But you could do a google search.