What Could Red Bumps On Stomach, Chest And Back Indicate?

About two weeks ago my 3 year old son developed about 25 small read bumps mostly on his stomach and chest, and a few on his sides and back. They look like very tiny pimples. They gradually (and quickly) develop into a white head and pop. Once gone they leave a blotchy darkend discoloration. Then new bumps develop. He doesn't scratch them and he didn't seem to notice they were there until I discovered them.