Try scrolling to the right more under the "Friends" section instead of looking under the "Passerby" section. If you don't see him, try deleting and registering his friend code again.
>For him to appear in the friends section, I think he needs to be online playing the game at the same time as you. That might be it.
commented 9 minutes ago by GeoTruth
As GeoTruth said, he may need to be online at the same time as you for him the appear under the friend section.
Hope this helps!