One word of warning.....don't know if they're doing this now, but last year I remember reading many threads where every member of the party had to appear at the check in counter, including all children, during
FREE dining. I guess this was to prevent people from having "phantom guests" to get more dining credits or something equally stupid, lol.
I haven't read about that this year, or maybe because it was so prevelant last year people have accepted the practice. I believe that what was being done was if some of your party arrived later than the person checking in, their free dining credits weren't being put on the room keys until they arrived and checked in.
Now...if you're talking about paying for the dining plan....then no problem, they don't care since it's paid for. The credits are not person specific (except adult/child divisions)...but if it's 3 adults then you would simply have 3 TS, 3 CS and 3 snack credits for each day (so in your case 24 TS/CS/snack)....how you spend them is up to you. So the two who arrive first could definitely be doing several Signature meals that cost 2 TS, or character breakfasts and sit down dinners, etc. And eating yourself silly on CS and snacks, lol.
BUT....might it be cheaper to book him his own room/dining plan for ONLY the one day he'll be there?
Considering that the dining plan is $38 per person, x 7 days he wouldn't be using it, that would be enough to pay for many of deluxe rooms for a night! If you're staying value or moderate, I'd definitely book him his own room and dining plan for only the dates he'll be there. If he doesn't want to stay in his own room and bunk with you guys, that's your business, but financially it'd be cheaper to do his own.