Actually, it's 3 years old rather than 2, when they start getting charged.
OP--I'm guessing you're talking about adding him for free dining?
In that case, I think one problem you may have is that they probably have him already in the system as one years old. I would imagine they would get suspicious if suddenly he is three....during free dining. Also, if he is three now, he'll always be two years older in their records. So if you're planning on going back in a year, you might end up having to buy him park tickets. Another issue, supposedly during free dining everyone in the party needs to be present at check in. Will your 18 month old pass for a three year old? PLUS even with the free dining--it still probably would be cheaper to pay out of pocket, and not have to buy the park tickets. The kids meals are pretty cheap; I think even at TS restaurants they are only like $7.00 or 8.00. And remember at buffets and family-style meals kids under 3 eat free! So if you have any of those, you'll be set!
And--for park tickets, everyone in your package has to have the same park tickets. So, for instance, if you have AP's or 10 day tickets or whatever, your son what have to get those tickets too in order to add
DDP. So it wouldn't work to buy him just a one-day park ticket, and then get him into the parks free the rest of the time. So, financially, you'd be much better off just buying his meals OOP.