apple Id

I don’t have access to his email 

You can't have access to others iPhone, email & others.

Ask you brother to read:

(< iOS 15.2) See this Apple article: If you forgot your iPhone passcode

If you forgot your iPhone passcode - Apple Support

(iOS 15.2 and newer) How to reset your iPhone, iPad or iPod touch if you've forgotten your passcode

How to reset your iPhone, iPad or iPod touch if you've forgotten your passcode – Apple Support (UK)

Have you forgotten your Apple ID?, Check your Apple ID via your email box, see this Apple article: If you forgot your Apple ID

If you forgot your Apple ID email address or phone number - Apple Support                  

OR see -> Having trouble signing in?

https://iforgot.apple.com/password/verify/appleid

If you've forgotten your Apple ID email address or phone number

If you've forgotten your Apple ID email address or phone number – Apple Support (UK)

Have you forgotten your password? -> See this Apple article: If you forgot your Apple ID password

If you forgot your Apple ID password - Apple Support

Then see this Apple article: What to do after you change your Apple ID or password

What to do after you change your Apple ID or password - Apple Support

He doesn’t know the security questions. 

See -> Get a verification code and sign in with two-factor authentication

Get a verification code and sign in with two-factor authentication - Apple Support

For the security questions, if you are being told there isn't enough information to reset them, contact Apple ID Support.

If you don't know the answers, if the steps in the article below don’t work, contact Apple ID Support, although based on multiple posts I have read, they will not help you recover the answers to your security questions.

 Apple ID - If you forgot the answers to your Apple ID security questions.      *

From the article below. 

Without proof of your identity via security questions and other carefully selected criteria, Apple Support can't help you reset a password or perform any other actions on your account. These policies are audited and reviewed on a regular basis.

Security and your Apple ID.    *

Apple ID - Official Apple Support.    -

Contact Apple for help with Apple ID account security - Apple Support.  * (Link doesn't always load)  

Get Support   - Work through the screens to get a chat option. 

Apple Support schedule phone call. -

Apple on Twitter