This question comes up alot and I know that some people bring copies of birth certificates with them. I don't think it's actually necessary though as I can't see a Disney employee actually arguing with you about a child's birthdate. When my youngest was 2 1/2 he also looked much older. We were in and out of the parks for 7 days and only once did a CM ask if we had a ticket for him. We simply replied that he was 2 and the CM said, "ok great, have a magical day", no further questions asked.
On the flip side, we were once putting our other son's ticket through when he was about 3 1/2 and a cast member kept telling us he didn't need a ticket.
Have a great trip!