You need to re-acquaint him with boys/men ASAP, so he doesn't have this problem for the rest of his life. Your brother's friend was wrong, but that doesn't need to scar him forever. Give whichever brother you choose a few treats to give Chewy, and lead him gently over to that brother, and let your brother reward him with a treat for being a good boy. Maybe your brother should pet him, than give him another treat. Make sure your brothers are extra-gentle toward him for a little while, until he understands that all boys are not that mean boy!