Hi there, Rachael! Thanks for bringing your question to the Disney Parks Moms Panel!
You are correct, children under the age of 3 do not require a
ticket to enjoy the parks at the Disneyland Resort. Also, proof of age is not required. You can bring along a copy of his birth certificate if you are concerned, but I can almost guarantee you won't need it. Cast Members generally take parents at their word when asked about their child's age.
Your little one is going to have a blast next summer! I suggest measuring his height before you visit so you will know if he can ride
attractions like Autopia, Luigi's Rollickin' Roadsters, and Mater's Junkyard Jamboree. These attractions have 32" (81cm) height requirements and when my daughter was around your son's age she was able to ride these attractions and loved every minute! If he doesn't meet that height requirement, fear not! There are a multitude of attractions that have no height restrictions that he can enjoy with you.
Please return to the Disney Parks Moms Panel with any other questions you may have. Happy planning!
-Linda M.