If you know his username on ArmorGames, use the URL [http://armorgames.com/user/] and paste the username at the end. This will bring you to his profile. Inside the profile, there will be a big button with green font "Add Friend". Click that, and it will be added to your profile friends' list, where you have easy access to his profile.
And remember, you will add him as a friend, but he is not required to add you. There is no friend request thing like on Facebook, but in AG3, you will have "followers", who are people who added you, and "friends" who you've accepted as a friend.
