We did kayaking this past March and had a lot of fun doing it. They are double kayaks so your son would have to go with an adult. It's milder weather than the summer months, which IMO, would be way too hot to be doing the kayaks.
Bring or buy a kite to fly on the beach. There is a kite store in Caligny Plaza which sells the easy flier kites. My kids love going out to the beach pre-sunset.
We love the rent bikes. HH is very bike friendly with a lot of paths all over the island. You can bike as far as you like. We love to ride over to Cold Stone Creamery.
In March we were in need of bug spray at Hilton Head. Be sure and bring it along and apply it liberally. The no-see-ums, or sand flies, were terrible.
My kids wouldn't swim in the pools, because they were cold, but my DS7 enjoyed swimming in the hot tub. As no one else was brave enough to get in it, it was the perfect size for just him.
ETA: The crab may not be biting yet for the season, but if you have a fishing pole, you can try to fish off the pier at the resort. My kids were able to catch a couple of small fish there this past spring.
Have fun!