A messy situation without a simple solution. It sounds like he made the mistake of creating a new Apple Account instead of simply changing the email address on his old one. Purchases are permanently tied to the ID that was used to download them originally. His options now are to either try to recover access to the old ID and use Family Sharing to share them to his new ID, or to delete the apps and redownload them with his current account. This will entail having to repurchase ones that cost money, and possibly losing data such as game scores.
If he want to go with trying to recover access to the old account, click on this link to read what to do if you forgot your Apple ID [Account] password --> If you forgot your Apple ID password - Apple Support
or go directly to this link --> http://iforgot.apple.com/
Also read about "What to do after [or before] you change your Apple ID email address or password" --> https://support.apple.com/HT204071
If you didn't receive your verification or reset email, read this Apple Support article: If you didn't receive your verification or reset email - Apple Support
"When you make certain updates to your Apple ID, we’ll send you an email to help you complete the update. For example, you might have reset your password, reset your security questions, or added a new email address to your account."
If you can’t sign in, access a trusted device, reset your password, or receive verification codes, you can request account recovery to regain access to your account. This article explains how to recover your Apple ID after a waiting period - How to use account recovery when you can’t reset your Apple ID password - Apple Support