First, your estimate of water requirement is off. 8 pounds of water a day is off. He will do fine on half that - 4 pounds of water a day - that's 64 fluid ounces.
He can produce that much water with a human powered RO device, probably integrated with his pedal system. His problem will start if his RO apparatus fails or even starts working inefficiently. I'd be surprised if he doesn't have a backup.
I don’t think you’re correct on the water requirement.
I allowed a gallon per day. Your 1/2 gallon might be ok for water only, but not IMO if all food is dehydrated, as it obviously is if he’s surviving on a little more than a pound per day.
I’ve had some experience in food and water requirements due to experience in desert backpacking. I used to carry a minimum of 2 to 3 quarts per day, but that was generally with undehydrated food.
In hot or dry weather much more.
Here’s a water requirements calculator.
http://www.csgnetwork.com/humanh2owater.html
I’m not familiar with water purification equipment of the type you mention. There is a feedback factor here. The more power needed to purify the seawater, the more he’ll need to work to produce the power, which means the more water he’ll need.
I’ve read Kon-Tiki, and from their experience it looks like, at least in tropical waters, he won’t have any problem with getting enough food to survive.