Some notes:
a) Disc games work on whichever PS4 they are in. Unsurprisingly.
b) PSN won't let you sign in on two PS4s at once. So he won't be able to use your account at the same time you do.
c) Games bought from PSN (including free PS+ games) always work for the PSN account that buys them, on any PS4. You can play your stuff on both consoles.
d) Each PSN account has a "primary" PS4. On the primary PS4, and only that one, content from that PSN account works for all signed-in users. That's why your son can play your PSN purchases under his own PSN account today; you have one PS4 and that's your primary. You can change which console is your primary at any time through Settings on the PS4. When you get a second PS4, one of them is not going to work for him; he will not be able to play PSN content owned by your PSN account on one of your two PS4s. You'll have to pick which one and/or re-buy some PSN games under his account and/or buy games on disc (but even then, DLC is still PSN content, so that's not a total solution.)
e) Note that your son can also use your PS+ account's ability to play online under his PSN account when signed into your primary PS4. He doesn't need your account to play FIFA online. More:
https://www.playstation.com/en-gb/get-h ... station-4/