Right, I've e-mailed Epic but in the meantime I've logged in to my account that is linked to my PSN account on the website and if I go to "Connections" I can see that it is linked to my PSN account and there is a button to "Disconnect" - could it be as simple as clicking this button and then booting into the game under my son's PSN account and simply linking the two from there? (obviously I don't want him to lose all of his progress and the battle pass by doing this!)
*edit*
Also, I'd doing all this on the assumption that he will still be able to play online even though it's my account that has the PS+ subscription? (I've logged into Battlefront 2 and managed to play online under his login so I assume it will be the same?)