Teenagers are learning how to navigate relationships and may not understand that some girls simply want to be friends. People have a tendency to take things personally. Tell him that it wasn't easy for you to open up about this, but that it was necessary. Based on some of the things said to me during my teen years and to my friends. Having just started a new job, I don't want any misunderstandings between us. Just keep hanging out with your group of friends, and be nice when you see him.
You don't have to go overboard.
How do I tell a guy I’m not interested and just want to be friends without hurting his ego?
Just the usual banter as with others of our group. Don't be wishy-washy about it. I made note of the likely exceptions, and what that may look like. Just tell the person. There's no light-hearted way. After at least a couple of months, try hanging out in a platonic situation.