Hi Janice! How exciting to take your Grandson on his first Walt Disney World Resort Vacation! You are an awesome Grandmother!!
It's A Small World at Magic Kingdom Park does not have any height restrictions. Toy Story Mania! at Disney's Hollywood Studios Park does not have any height restrictions, either. However, Soarin' at Epcot does have the height restriction of 40 inches or taller.
I suggest that you visit these three Parks and if you have time, Disney's Animal Kingdom Park also. I would suggest that you visit
www.disneyworld.com and go to Things to Do and select attractions. You will be able to look at the attractions for your Grandson's age by selecting his age under the recommended ages tab on the left side of the screen.
I suggest that you remember to make time for his nap and a little Rest & Relaxation mid-day. I also suggest that you bring snacks that he enjoys to help with lines at the attractions.
My biggest suggestion is to have fun and enjoy creating wonderful memories with your Grandson on his first Disney Vacation! Enjoy the Magic!