Crunched some numbers here.............
Could you simply book him as a 10 year old, pay the higher entry ticket price and higher DDP and then he could eat what he wants off the adult menu "legally"? It sounds like the extra expense of a ticket (for example, base 7 day ticket would be $37 more or $7.40 more a day), plus the extra $18 a day for the adult vs children dining plan would be worth it. That's about $25 extra a day, but otherwise you'll probably have his kid TS and CS going to waste, plus having to buy him the meal he wants out of pocket. Just one dinner in a TS is likely to run you more than that $25 a day, and the adult DDP would mean that he could have what he wanted at the CS as well.
Even if you wanted the more expensive tickets, the price difference isn't much....in fact the 7 day hopper is only $32 difference, so even less than the base difference....a 7 day everything is about $40 difference....spreading those over 7 days is still very close to the example I used above. Even an annual pass would only be $66 difference, spread over how many days in the year you might use it. So the ticket difference between kid/adult is pretty negligble when the difference is your son's enjoyment of his meals.
While researching whether the DDP would be a bargain for us, I noted that most CS meals that we might want, with an appetizer/entree/dessert/drink would easily run $15 and in many places our choices would be $20 or more per person...if you only did an entree and drink it'd be in the $10 range. TS would be at least $20 just for an entree, throwing in an appetizer/dessert/drink would almost double it. So you'd likely spend $30-$40 a day easily on buying your son the food he'd prefer....plus you'd have a kid CS and TS credit that you already paid for going to waste (you could use the snack credit still, but CS/TS would be worth $16 a day of a kids DDP cost).
I know someone may say this is wrong to lie about his age....but hey, at least the lie is in the right direction, in other words, you aren't trying to get a LOWER price by lying, so while it's a little grey in the integrity arena, I'd probably be quite comfortable doing it since it benefits everyone....Disney gets more money, your son gets to enjoy his meals. The wrong is when a 10 year old is said to be 9 so you save a few dollars, IMHO.
My understanding is that whether you say he's 9 or 10 he would still be free in the hotel room, so there wouldn't be the "extra adult" expense there, but you might want to be sure it's the same whereever you are staying.
I'm glad that my girls (age 11 when we go) will be "adults" because I know they'd be bored and unhappy eating nuggets and burgers and the same boring desserts and then throw in watching me dine on a wide variety of entrees (including steak) and an equally wide variety of yummy desserts....... I'm not sure our vacation would be as pleasant if that happened!
EDITED: Ok....I had a thought and wanted to be sure to bring this up....IF you are going during the
FREE dining plan period (September I think)....then "I" would not be comfortable lying about his age because then Disney
would be losing....they'd be providing you with an adult DDP for free, when he's really only entitled to the free child. BUT....if that were the case, I'd take the free child's DDP for him, throw away any of the CS/TS that he doesn't want the kid's meal and pay out of pocket for the meals he wants adult menu choices.....considering that you (and any other adults in your party) would be getting free meals this wouldn't break the bank for me, especially if it's how I'd make my child happy at mealtime. Oh...and if by chance you are going at free dining....from what I've read almost all ADR's everywhere are booked solid for that time period, so you may be out of luck being able to make advance plans....and from what some are saying, you may go hungry because the free days are almost impossible to get walk up reservations at most places (seems some resort restaurants it's possible). In that case, I personally would probably change my vacation dates, because while food isn't all I am going to WDW for.....I wouldn't want to be brought off my cloud by having to go off site to eat for every meal! That'd be a real downer (of course that's why "I" go during off season, less downer moments!)
Jan
I offered to pay for the adult meals but, the CM's all let my kids get adult CS meals with the DDP. I don't think they allow that anymore, so we are planning accordingly for our next trip. 
