How an i get my sons profile back onto my profile?

Hello all

I stupidly tried a reset to get my son onto the fortnite live event but once it booted back up his profile was missing from my home screen.

We can't remember any login details for his account and I have tried to contact Microsoft but don't know how to get anyone to get in touch with me.

We spent a lot of money on all his games etc and don't want to loose all his achievements etc.

Best regards

James

:)