Child turning 3 during trip

[Katy2605]

We are hoping to book our 2020 trip on Thursday (all being well!) for an onsite Disney trip for 10 days. At check-in my son will be 2 and will turn 3 4-days after check-in. From all I have read I was under the impression that Disney will deem him to be 2 for the whole time of our stay so I won't have to pay for him to have a Disney ticket. I started to do a dummy run booking and on the disney booking site it asks for the age at time of return travel which would obviously be 3.

I rung disney to clarify and I think I confused her, she began by saying I need to put 3 and then changed her mind and said to put 2. I wondered if anyone here could confirm what to do? I am happy to put 2 but obviously that is not technically true. I guess if I do have to put 3 that would mean he will get "free" disney dining!

Thank you for any help!
Katy