My son will be turning 3 just a few weeks before we go to disney. He does not eat anything and I know the dining plan will be a waste for him. Do you think I can get away with passing him for 2? He is tall for his age so he definitely looks 3.
To answer your question, yes, you probably COULD get away with it.
However, (and I suspect you must have known there was going to be a however), doing so is stealing. Also, since everybody in your room has to be on the
DDP, in order to avoid buying it for him you'd have to list him on the reservation as being two years old. Then you wouldn't have park tickets for him, so you'd have to sneak him into the parks, as well, and that's another pretty big deal.
The dining plan for a child is $12 per day. It's not a huge cost, and it's the price of poker at WDW. If you really think he won't eat and can subsist solely off of your plate for the duration of your vacation, then the whole family can forego the dining plan and pay OOP for everybody. However, if you go to any buffets/family-style meals, you'll still have to pay the child price for him, since he's over 3, whether he eats much or not.
I realize three is an arbitrary cut-off and for some children can seem like a huge waste of money, but it's the age WDW set for kids to begin paying and so choosing to do otherwise isn't right, in my opinion.