WDW is very consistent on the DDP. Even families with a family member who is tube fed and doesn't eat anything at all have to pay for the plan. The best way to not pay would be like the agent said waiting and adding him at check in which works totally fine as long as you aren't using magic express. They stopped doing the entertainment fee at character meals so you would have to pay full price for those. We have not been charged once when my child ate 2 bites and fell asleep but that's really unusual. You can email special diets and they will try and accommodate when they can but it's not normally down to specific brands of syrup. They also can not heat up items you bring with you as there is some legal liability so if you bring the pizza or spaghettios to a restaurant he would have to eat them room temp.
just a note on this... the resort food courts do have microwaves... if you need to have the spag-os heated, perhaps bring a thermos and heat them in the morning and carry them heated for a quick meal whenever ds opts to enjoy them...
i think i said it above, but agree w/the others that if you opt for the DDP for your son, you will get your $s worth just w/snack, CS (share amongst those that opt to eat) and not having to go OOP for the character meals...
however, the bottom line is that you ENJOY! and have a MAGICAL VACATION!
But if you think he will eat even $13 a day in snacks and milkshakes, then it might be worth it.