Try a short Ka-Bar with a leather handle. The blade is carbon steel, which means it will rust. However, teach your son to properly care for a knife and that will not be a problem. If you search around, the short Ka-Bar can be found for $40-$50. It is a great investment, as the knife is very well made, and the carbon steel takes a great edge. The leather handle will fit well in your son's hand, and not cause discomfort after long use.
Above is a picture of the short Ka-Bar
If you cannot find a Ka-Bar, try a pilot survival knife by Camillus. This knife is similar to the Ka-Bar, but has a thicker guard, and a toothed spine for notching. It is also a carbon steel blade. It costs about $35-$45.
Above is an image of the Camillus pilot survival knife
The Buck 119 Special is well-built and has a great sheath, but the blade steel is not of the best quality. I agree with most of the knives mentioned by others, with one exception: do not buy a Benchmade 10510 Gamer. This knife has a very thin, uncomfortable handle that would be painful to use for long. It also may be too wide for your son's hand, and the sheath is poorly constructed.
These are just some suggestions that I make based on personal experience. I hope you find a sturdy blade for your son.
TheSurvivalist