Well, I don't know much about your friend, so here are some ideas for different types of personalities:
Funny Guys: Jimbo Joe, Bilbo (The Hobbit), Paco
Smart Guys: Professor, Gandalf (Lord of the Rings)
Cool Guys: Dragon Claw Death Hammer, Reddington (Blacklist), Burt Macklin (Parks and Recreation)
Fitness Enthusiasts: Sprinter, Muscles, Jazz Hands
Serious Guys: Stoick, Batman
Think about what things he likes, if there's anything he's utterly obsessed with, whether it be a specific food, car, sports team, etc. Personality and appearance are also potential sources for a great nickname.
But make sure that your friend likes the nickname you choose! For example, if you gave him a kind of "goofy" nickname, it could hurt his feelings, and he may not even tell you about it because he doesn't want to make an awkward situation.
I would suggest saying "Hey, I was thinking of a nickname to call you. I really liked this one, and this is why. What do you think of it?"