Thank you for your reply.
I have tried to change the picture as per your suggestion for the Apple Account Photo.
I can log into my son's Apple Account with his iCloud email.
But when I go into Personal Information I can see the picture that we want to change (it is actually his mother's picture) but there is no way to change it on that page.
I have also tried to go into his iCloud settings and under 'Photo', it has 'Change Apple Account Photo'. It allows me to drag a new photo into position, zoom and rotate it but it doesnt Save it. I click Save, the window closes but the old photo is still on the profile.