There are people in my past with whom I was friends, but we grew apart. So what? As long as you've done nothing to hurt him along the way, if your friendship fades, that's a shame, but not a disaster, or anything that you are doing wrong. It's the same process as parents go through when their child grows up. Of course we should be reliable adults to our YFs - but our responsibility to theem is ultimately that of any adult; to be there for them, to give them appropriate space in our lives, to give them space to grow up, to offer them information about how the world works so they have a chance to avoid the more obvious mistakes that can be made.

And certainly worrying too much about the future is a big mistake - as Jesus comments. You might get hit by a truck tomorrow, therefore he is devastated by losing you. Therefore you shouldn't have been his AF because that might happen?!

Be a good friend to him. Be the best you can. Given that you are the adult in the relationship, you have additional responsibilities beyond that with an adult friend. But it's not a massive amount more.