an avid outdoorswoman once told me, "Cotton kills." In cool weather it sucks away any heat you've generated because it holds the sweat. the cheap alternative for under your waders is polyester fleece. doesn't have to be fancy; does have to be not cotton. polyester is the way to go because it will wick away the sweat.

after trying both kinds in winter, i found the neoprenes just not warm enough. breathables were the most comfortable, with longjohns and fleece underneath. wool socks are essential.

don't know how big your boy is, but my first outfit came from Bass Pro, stocking foot with separate boots, recommended as better for walking in to the fishing. even bigger kid sizes are cheaper than adult stuff, dunno why.

oh, and make sure to wear fleece and/or wool on top too. some folks just are colder than others (especially if they are smaller) and might need a stocking cap or even better, a neck gaiter. that little extra around your neck is as good as an extra sweater.