They are a little different from the hard plastic ones, but they are the same size - I think your son will be fine - they are really meant for young children, not babies!! We rented one for my daughter last December, and she was 6 and fairly tall for her age (48 inches on the nose - she got to ride Rock and Roller Coaster- but skinny. She fit just fine. It's more of a choice whether you will need one based on your son's energy level - MGM is a little more compact park - We definitely needed a stroller at Epcot - not necessary at the other parks, but she wasn't feeling well, so we saved all her walking energy to get her through the day.