You might want to check out the forums at
www.fosterparents.com there are many adoptive parents who have dealt with the interstate issue.
It's my understanding that since he is already adopted, that CA would accept the medicaid, you shouldn't have to do an interstate thing since you've already adopted him. Who told you that?
I'd also try contacting CA's medi-cal program
http://www.dhs.ca.gov/mcs/medi-calhome/FAQs2.htm I believe this is what he would qualify under...if you are receiving adoption assistance payments, it appears that he qualifys.
Good luck!