To answer your question: no. You can have multiple service accounts on one account, but the account can only be logged in to one platform at a time. Trying to log into a different platform will cause the other connection to force terminate.

If you & your son want to play together and the ability to play on the different platforms, you will have to have two separate accounts, which will require 1 PC license & 1 PS4/5 license each, unfortunately.