Well, in a normal year, the weather in March is fairly warm - shorts and tee shirt weather. However, in this really odd year we’ve been having, you never know. You probably want to throw some jeans and a sweatshirt just in case.
I recommend a backpack - not a full-size one, but a small version (draw strings are good) will work. Fill it with: water bottle, snacks, mints/gum, hand sanitizer, mini first aid kit (ibuprofen, neosporin, anti-itch cream, band-aids, dramamine, chapstick), sunscreen, camera and batteries, pen/pencil, autograph book if your son intends to collect autographs
As for ME, never used it, but it looks like Zefyr’s got you covered.
Carry-on… for the plane? Well, other than typical carry-on items (magazines, iPod, etc.), many suggest throwing in swimsuits in case you want to go to the pool once you arrive… maybe flip-flops, as well. That’s about all I can think of.
Have fun at WDW!