If you have status, then you can have someone sit in E+ with you regardless of whether they are on the same PNR; just call to get him moved up. If you just have the E+ Annual, he must be on the same PNR as you.

The other (non-) issue you will encounter is that when you try to buy a ticket just for him, .bomb won't let you buy a ticket for just a 5yo (and no adult traveling with him). I have then entered Master PVDProf as an adult, booked the ticket, and called to have it noted that he and I are traveling together (though this doesn't really do anything). I sometimes worry this approach may in fact not provide much protection for keeping the two of us together in irrops, and therefore there's 1) a reasonable chance I'd have to tell an agent that I'd take whatever irrops option my GM son is given, rather than what I'd normally be offered as 1K (e.g., a UA flight in three days, rather than the CO flight leaving in an hour) and 2) some small chance they'd tell me touch luck with that and I'd have to buy us two tickets on the same PNR later.