Questions on possible options

Hi everyone, I know the answer to my question is adoption is probably not an option but I figured it could not hurt to ask. I have become very involved in a sixteen year olds life. He looks at me and thinks of me as his dad. I love him as if he was my own flesh and blood. He lives with his mom. His biological dad has never been involved in his life. I am friends with his mom and she loves the fact that he now has a dad in his life. Lately I have been wondering if there is a way to make it legal. I know adoption usually severs ties with biological parents. That is obviously not an option because his mom is part of his life. His real dad there would be no problem with severing ties with since he has never been part of my sons life. I know we will have this father son relationship no matter what for the rest of our lives whether its legal on paper or not. There are some advantages obviously for it being legal. One his mom has no benefits and he has no health insurance. I would love to be able to put him under my insurance. Two I already deal with his academic side. His teachers never ask who I am since he tells everyone that I am his dad but it would be nice if it was legal. His mom has no problem with me being involved with his school affairs. If anyone has advice or knows if there any options for me it would be greatly appreciated Thanks, Mike