How does my son use his profile on another Xbox?

Hey there!

You will need to promote his child account to an adult account.

After you reach the age of adulthood (which varies by country and region), you can promote a child account to an adult account. This promotion removes the Family Settings and Xbox Live restrictions.

To promote a child account to an adult account

• Sign in to Xbox Live by using the account that you want to change.
• Go to Settings and then select Account.
• Select Promote Child Account.
  Note This option is available only if the date of birth for the account indicates that you have reached the age of adulthood (18 in the United Kingdom). The legal adult age varies by country and region. The age of an account owner is calculated
  by using the date of birth provided when the account was created. This date cannot be changed. If you entered an incorrect date of birth when you created the account, your only option is to create a new account using the correct date of birth.
  
• Follow the on-screen instructions to promote the account.

question ... it suggest I need to remove him from the family account but it says that ... "he will still have a Microsoft account but won't be able to use Xbox and Xbox services until they are added to another family"

I don't understand what this means.

Does it mean he will be able to sign on another Xbox while down at university or not?

Yes he should be able to sign into another Xbox, although he will not have any Xbox services such as Xbox Live or Xbox Game Pass. He will have to purchase these separately from your Xbox account.

You should be able to connect his profile to a separate Microsoft account all toer.

Does that mean a paid separate account?