I think you'll find most high schools will require that on challenge day, both wrestlers should be no more than 5 lbs over the weight class they are challenging for. I've had several coaches tell me this 5 lb figure, so I assume it is pretty common. So if your son is weighing 99, he could be wrestling a 111 lb kid.
I don't believe there is any rule that says the kids have to be within so much, and if there was, it would probably disallow the one that is too light, to wrestle.
I think we had an issue with a kid in the heavyweight class in the kids' series a few years ago that signed up for heavyweight, but didn't weigh enough to compete in that weight class.
Maybe someone else can clarify...