Hi, Nikki!
Both Blizzard Beach and Typhoon Lagoon will be a blast for your son! Just out of personal preference, my family enjoys
Blizzard Beach more. My kids love taking the chair lift and they could spend all day in the wave pool!
The temperatures in March should be somewhere in the 70's. For my kids after a winter in Ohio, that is a heat wave! But you will have to be a judge of that. You just can't tell what the crazy weather is going to do - today in Orlando, end of February, it is going to be near 90 degrees! Then by tomorrow it will top out around 70. You will have to take the weather day-by-day to see what the forecast holds.
What you might want to do a day or two before you leave for your magical Walt Disney World vacation is to check the upcoming forecast. Pick whichever day is going to be the warmest and use that day as your waterpark day (hoping this will not interfere with dining reservations or prior plans!)
I hope your son has a great time at whichever water park you end up choosing. Take your sunscreen, water shoes or flip flops and have a "splashing" good time!