You can have up to 8 service accounts on a Square Enix account. They will all use the same login credentials, you switch by selecting "Data Center" from the main menu of XIV, but each service account is a separate subscription and has separate game licences. Examples:

Service Account 1: PC, PS4
Service Account 2: PC, Mac
Service Account 3: PC

If you have an active subscription for each, you can login all three accounts at the same time, but only one character from each and only on the platforms that there is a licence for.

So from the example above you could could play on PS4 on account 1 and PC on accounts 1, 2 and 3, but you couldn't play on PS4 on account 3 as it doesn't have a PS4 licence. Although account 1 has a licence for PC and PS4 you can only login on one at a time.

You also need to have the expansion content as well. If you tried to log into a character that was in a Stormblood area, but you don't have Stormblood, it won't let you login. You'd have to login using the platform that does have Stormblood, move your character, then it'd let you login on the platform that doesn't have Stormblood.

However, saying all that, if the service accounts are for different people I really do recommend you do not create them on the same account as they can never be split to a different account. Likewise characters cannot be transferred between service accounts, even ones on the same main account.

I don't know how old your son is now, but even if you're the one controlling both accounts for now, if you create his account as a separate account then one day you could at least hand the account/over to him when he's old enough. If he's created on your account, you'll have to continuously share login details (which technically breaks the terms of service if he's no longer a child etc) and that'll cause problems when trying to use two factor authentication too or one of your would have to start over.