If those accounts were opened in the past 24 months, Chase's automated process will count them the same as if he had opened them. When he receives the letter he can call and explain that he is an Authorized User on those accounts, and responsible for payment. The agent can manually adjust for those.
If that's not the problem, then perhaps he needs to look at his credit report to see if there is any unauthorized activity.
Very grateful to you for responding, Mia.
I will follow through with your suggestions and report back.
Thank you!