Hi Dina,
Thanks so much for visiting the Disney Parks Moms Panel with your question. With a 5-year-old who is 42 inches tall, he will make the minimum height requirements for most rides, but there are still a few for which he will not make the minimum height.
To find out the height requirements for attractions, you can visit the
attractions page for Walt Disney World. From here, use the filters to set the height requirements that your son will not make - so you would select 44 inches or taller and 48 inches or taller (the only categories above 42 inches). This will give you a list of attractions that your son would not be able to ride at Walt Disney World. You can also visit the unofficial site allears.net and use their search tool to search for "height requirements." The first link will give you a list of many height requirements, as well as other information about rides, such as those which may be intense for younger children, in case you are interested in that information.
I hope that this information is helpful for you, Dina. Have a great time planning your vacation, and please do not hesitate to ask if you have any further questions.