Not sure how far you guys wanna drive.. but I've been to Florida a lot so I can give you some suggestions...
Cocoa Beach is closest to Disney and you could also go to NASA if you wanna see some space stuff... your son might like that. At cocoa beach there's a really fun beach bar called "Coconuts" and you can buy drinks and they give them to you in plastic cups and you can chill outside or drink them on the beach. There's also lots of shopping and probably other activities, I spent most my time on the beach though.
I've been to Florida in Feb and the water was warm enough to swim in.. it just depends on how lucky you get.
As for the gulf side..I was never a huge fan because I grew up going to the Atlantic but the gulf side is nice too.. they have white sand beaches but I've also heard they have more sharks!

It's been my experience that the gulf-side is more for an older crowd but I've only really been to Naples on that side.
If you have the time.. I'd personally go farther south to Boca Raton/Hollywood/Fort Lauderdale/Miami. You can get some pretty good deals.. my grandma had a condo right on the ocean in Boca.. it was beautiful. Also, on the drive from Orlando down to the southern atlantic coast you can stop at Lion Country Safari.. really fun- you basically drive through a "safari" of real animals in your car, you can also feed a giraffe, ride a camel, etc. I did it like 2 years ago when I was 21 and still had fun. I LOVED Miami beach, it was surprisingly clear and awesome for it being in a big city. If you go to Miami you also could go to this Parrot Island thing.. you can interact with the birds and stuff, I think that's the name but I can't remember... and def eat at Pink's...
Don't bother with the Florida Keys.. all man-made beaches and every time I've been there it's been a disappointment. But an airboat ride in the everglades is really fun!
Hope that helps!