Over use

Last summer I took a class in "Life Span Development" and I remember reading that when children learn languages it is more common for them to under use a certain word then to use it in too many cases...

I am not so sure that is true for Doug. Here are a couple of his common over uses:
Acorn= he uses it for acorns, popcorn, and corn as in corn on the cob
Soccer Ball= anything...and I do mean anything that is a ball shape is a "soccer ball"...no matter how much I correct him or model for him. Any ball (basket ball, football, baseball, etc.) is a soccer ball and anything that looks like a ball such as the meatballs on his plate of spaghetti tonight.