They probably think of childhood friends as pseudo-siblings, but many people don't agree. Your experience of crushing on a friend is common so don't let your other friends shame you about it. You just happen to have been friends with him for many years.
You can try to deny feelings, get over them, or keep them private, but they are what they are.
Both your feelings and perceptions of him have evolved, and telling him will alter your relationship regardless of his response. As long as you realize that, why not talk to your crush about your changing view of him without pouring out undying romantic love? You might, for instance, describe how you no longer think of him as the same little kid you climbed trees with once, remarking that he's grown into quite a handsome guy. Add a little flirtation and inside humor to it, then gauge his reaction. That will let you know whether he's looking at you in a similar way.