According to Disney' dining plan brochures, you're not allowed to buy meals for people who are not on the dining plan with you. In practice, the room card shows how many people are on the plan (so it will show 2 A 1 C, meaning 2 adults, 1 child) and the servers will look at that and won't ring up more than 2 adult, 1 child meals at a time. You will occasionally read of someone being allowed to circumvent this, but it's not supposed to happen...and I would not count on being able to do that. Please note that many of the Disney reservation CMs are not really familiar with the ins & outs of the dining plan...many of them have never even been to Disney World (call centers are not located in the Orlando area...many of them are actually over in Tampa, 90 minutes away.) The CMs are notorious for giving totally incorrect information. The actual Disney Dining brochure pdf is available online, you can read the details of the plan yourself. It plainly states you cannot purchase meals for others using your credits.
I am curious about this situation, though...i understand that you could have leftover tickets from a prior trip, but to get a package deal with dining, you have to have everyone in the room on the same package. So if you registered him to the room, then you had to buy him park tickets on the same package as the rest of the family & he's listed on the free dining so you'll have credits for him too.
If you did not register him to the room (so he doesn't exist as far as Disney knows) he will not be able to use the Magic Express from the airport (Disney shuttle bus...not registered to the room=not not registered for the ME-he will not be allowed on if he's not on the list), and then also he will not get a Key to the World card (referred to as KTTW card on this forum) And without that card he will not be able to take advantage of any rides/attractions during the evening Extra Magic Hours with the rest of the family...well, he can stay in the park if already in it when the extra hours start, but he will not be able to get on rides since each person in the party has to show a current hotel room key card to get on any rides/attractions during Extra Magic evening hours. For the early morning extra magic hours, they won't even let him in the park without a hotel key card.