FC25 Online Connection - Switch

Can anyone help please? I bought an online switch subscription for £17.99 for the year, for my son's account. However, when my son tries to access FC25 online, it brings up the error saying you will not be able to access online features. Now I've realised that a  different Switch profile is linked to the EA FC25 account. I'm assuming that's the reason for the issues? Is there, however, any way that I can link his profile to the same EA account and he can then play online using his subscription? Or is the only other alternative to pay another £17.99 for the profile that is already linked to the EA account? I can see that you can unlink accounts, could I try unlinking the current profile and just linking my son's but can I do that as his account is set up as a child's account where I'm the supervisor etc. Sorry for the long winded message, but I was just wondering if there was some alternative rather than having to pay another £17.99 subscription.