help with setting up apple watch without previous apple id password

we have been given an iwatch by a friend for my son who's 12 - they were given it too and used it for a little while then put in a drawer for a couple of years - thing is they can't remember the apple id it was set up with or the password - they have taken it into the apple shop and they wouldn't help as they didn't have a receipt although they offered up their apple id etc - please help my son wants to use this watch but its just sitting here