If I become a legal guardian for my 16 year old nephew from England can he attend school here in the USA. Can I add him to my medical insurance?

Its going to take a LOT more than just that... they would have to have the right kind of VISA... and that's going to be the difficult, expensive and time consuming part.

Becoming a guardian doesn't grant him the legal right to reside in the USA on its own..

Your nephew cannot get immigrant visa. He will need to apply for student visa (F-1) and needs to go to private school only. Public schools cannot issue I-20.

All international students are required to have health insurance, paid by parents or legal guardian, etc.