B.G.
My mom (masters in early childhood education) explained to me that this is simply children explore the world.
I was going to say he might be doing this significantly less by the time he's three. I don't think my 3 1/2 year old does it nearly as much as he used it.
My husband and I usually say, "Oooh, yucky!" but that doesn't always stop him. Try not to worry about it too much. Most of what he's getting is just good, healthy dirt, as opposed to really dangerous germs. Think of it helping his growing immune system.
I would give it another year or so. But he really will grow out of it.