We were just at Water Country last Thursday, Friday and Saturday. My youngest is 8, and she was way too big/old for Kritter Korral. Your son is the perfect age. The water is very shallow, and everything is geared for small children. Maybe the 50" rule is the maximum for the tiny slides?
Also, the park has over 15,000 life jackets available (a guard told me exactly how many!). They are the Coast Guard approved foam ones that clip in the front. They come in every size from infant to XXL adult. They are everywhere! Just about everyone in the wave pool was wearing one because the waves got so rough. If you put one on your son, he'll be able to do Hubba Hubba Highway (lazy river), as well as the shallow end of the wave pool (the waves are VERY gentle there and it's zero depth entry) as long as you hang on to him. Also, since the lazy river doesn't have innertubes like WDW, most people were laying on the life jackets and floating around. We saw many, many infants and toddlers floating around the pool with the lifejackets, especially when the waves were turned off (They are 8 minutes on, 8 minutes off, with a huge digital counter in the front so you know exactly when the waves will start again).
The water was also pretty warm. We were there on a very hot day, and I wished the water had been a little cooler. The next day was only about 75 degrees, and the water still felt warm to me.
If you have any other questions, please let me know.