That Super Nelson thingy looks hilarious! Plus near impossible to use. Looks like you'd have to breath through your nose. Then, if you could submerge to the ends of the breathing tube, water pressure would prevent you from inhaling. I know. I've tried breathing underwater from a garden hose. Doesn't work.
Reminds me of my very first mask. Just like Spectrum's. Two tubes with ping-pong balls that were supposed to seal the tubes upon submersion. Also didn't work.
Really, just what Walter said. Mask, fins, snorkel, and twenty minutes of instruction, and he'll be a fish. Teach him how to flip his legs up to submerge, equalize his ears, and clear the snorkel the last couple feet on the way up.
You won't be able to keep him out of the water. When you do get him out of the water, his lips will be blue and his fingers will be wrinkled, but he'll be grinning ear to ear, and begging for scuba lessons.
Another option to consider is maybe a snorkeling vest. Just an inflatable vest that you blow up yourself in case you get tired. I saw those used in the Keys one time.