I'm not 100% sure, but I don't believe they have regulations on passenger age. Of course, he will have to wear a life jacket. I guess my biggest question would be, "Will he be able to see over the front end?" It'd be a bummer to sit there and look at the dashboard. Obviously, it would be unsafe for him to stand up during the ride. How about some type of booster seat?
The mouseboats are really fun! They were among my daughters' favorite things at Disney!