This weekend, we just watched our "daddy movies" (family movies) of when we took our 4 1/2 year old daughter and 1 1/2 year old son to DisneyWorld. Very fun! We have been there about a dozen times and my kids are now, 13 1/2 and 10 1/2 years old. We all have made wonderful memories there! You just have to know your son and his endurance level. At that age, you can rent a stroller at the park if you get in a bind and your child needs to ride in a stroller. We usually brougt an umbrella stroller, not for the reason of our child riding in it all day, but to cart some of our stuff around (i.e. cooler, rain gear, jackets depending on season and how long we were staying at the park, light snacks, water bottles, etc...) You can buy this stuff there, however, it just can get costly. At night, one of the kids usually rode in the stroller - we have spend up to 15 hours and 20 minutes at the parks, in one day. Sometimes they rode on Dad's shoulders; other times we went to shows and they took a little catnap. Yes, we had some melt downs but our rule has been, on a 10-day trip, we would try to go to the park for a full day - every other day or do some 1/2 day parks so that no one would get burnt out.
Just curious what time of the year you were considering??? There are lots of small rides to go on. However, my daughter at 3 1/2 years, went on the Tower of Terror and loved it (she STILL is crazy about wild rides!) My son is a bit more conservative. Depends on your child's thrill factor and what they will tolerate. He will not meet the height requirements for some of the larger rides but you can do a "parent-swap" feature of the rides, which means that one parent will stay in an area with him, closer to the front of the line, while your partner rides, then make a quick swap after the ride so that you both get to enjoy the ride and not have to wait in a long line again. Probably for the park that has the most rides for his height, I would go to Magic Kingdom. However, don't rule out EPCOT! My kids love it there. When you visit the countries, he'll be able to make a mask and add new embellishments by visiting each country. Each country has different shows and attractions that you can take advantage of - not to mention good food!
Nothing beats a 3 year old in a monorail, approaching Cinderella's castle,happily in awe, screaming, "Oh.... Mommy... Oh... Daddy... I see it! It's just like the pictures! It's a dream come true - I'm going to meet Cinderella and Mickey!" Yep, been there!!! Truly, it's a great age to bring a little one. My son's first trip there was 5 months old. It's a very physical trip - the type of trip that you may need a relaxing vacation AFTER your vacation but it definitely is worth it if you do a lot of advance planning and your expectations are (semi) realistic! If you are looking for specific advise, just Personal Message me.