No, you can't use the credits for anyone but the purchaser (unless everyone is staying in the same hotel room, booked on the same package,) but YMMV. That being said, you don't need to using the dining IN the parks, in fact, some of the best dining is outside of the parks. Boma is highly recommended. If you want character dining, 1900 park fare will be an amazing experience for dinner, especially if he shows up 'unattached.' I personally loved the Fulton's Crab house (would use 2 table credits) Or he could use multiple table credits for the days he's in the parks. Epcot has the best choices for these, including Le Celliers. *Dies at the memory of the steak...* And I don't even like steak!
Or, the better thing to do is not get the dining plan at all. Cost wise, it's usually very ineffective to use it for adults.