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I had this happen because I had not used credit in like 5 years. Mine didn't take a full 6 months to generate a score, as soon as my new card reported my score came back. However I still had paid off installment and revolving accounts (closed in good status, due to non-use) reporting. US Bank gave me a card - but I've had cards with them in the past.
Self Lender or an Alliant Credit builder account will add an installment loan to his credit profile.
I'm almost jealous because he gets to start from scratch. Don't get into trouble and learn from our mistakes! Read some of these posts and see how much a late payment or charge off can affect your life! While his score is high, he has a thin file (new) and it doesn't mean as much as someone with years of credit history. Take your time, but get some more cards. I would stay away from store cards. Check the pre-approval sites, but a few are not real pre-qualifiers. You can research the ones that are true like Capital One, Amex, and Chase I believe. Good luck!