I am not clear as to what you mean by he is in your iCloud. I don't think you mean Family Sharing because that would not let him get access to your passwords.
An account can only be set to one person at a time. You cannot add another person, hence my confusion at your statement. You can't have added him to your account. You cannot have had his birthday set unless what you actually did was changed your birthday on your account to his birthday. If you were sharing your account with him then change your password.
Read this document if you think your Apple ID has been compromised. --> If you think your Apple Account has been compromised - Apple Support
For iOS 15 or earlier, if you want to see if anyone else has access to your device or accounts, click here --> Checklist 1: Limit device and account access - Apple Support
For iOS 16 or later see how Safety Check on iPhone works to keep you safe. Safety Check for an iPhone with iOS 16 or later - Apple Support
Use the information in this document to check your Apple ID device list to find where you're signed in --> Check your Apple Account device list to find where you’re signed in - Apple Support
Related materials:
Personal Safety User Guide
Personal Safety User Guide - Apple Support
> open the Table of Contents and review the articles
A document with general information about security and your Apple ID --> Security and your Apple Account - Apple Support
Contact Apple for help with Apple ID account security. This page provides country-specific Apple Support contact information ➞ Contact Apple for support and service - Apple Support