I assume at this point he's past that having paid regularly on cards.I did this when I was first rebuilding my credit several years ago.
I took out the smallest loan I could from a bank (it might have even been a secured loan) for a 12 month term and just stuck the money in a figurative shoe box. I think I made 6 of the payments and then paid it off in full. I then took out another loan for a bit more and did the same thing.
It helped to build a good foundation.