When I invite someone to join the Choir that I manage, I follow the format instructions given on the website. This says only one person per line, in the following format:
email@example.com, Jane, Austin
firstname.lastname@example.org, Emily, Dickinson
It is asking that you put your son's email address followed by a comma and a space followed by his first name comma & space, and finally by his surname (with no following space and/or comma). The email address doesn't need to have a first name. In fact, the email address that I use is similar to your son's in that it is made up of my initials and surname at an Outlook account. And I have recently invited several new members that do not include the first name of the individual In their email address and these have been accepted.
I hope that this helps.
I find the best way is to ask your son to request membership then his 'needs approval' request appears in the 'Users & Permissions'
Then tap 'needs approval' and opposite his name tick either 'approve' or 'remove'.
You dont need to add anything.