Splash Mountain is actually a very gentle ride. It builds up the expectations of "thrilling", but it really isn't.
Gravity will keep your child in place. They don't need restraints. In fact, most coasters don't either - but they do have them in case a coaster gets stuck (if you need a restraint to keep you in under normal circumstances there is a problem with the ride).
The reason there are no restraints, besides the fact they aren't necessary, is because if by some fluke one over turned (and I've never ever heard of it happening), it *is* a water ride, so you'd not want to be restrained into a boat underwater.
There is nothing to worry about; if he meets the height-requirement, he's safe. The only reason it's not safe for smaller kids is not because they might fall out, but because they wouldn't be able to sit high enough in the log.
NED