What Causes Red Rashes With White Heads On The Buttock Of The Child?

My 8 year old son has a red rah on his butt, it looks like tiny pimples and a couple have what appear to be white heads on them, the spots stop at the entrance to the butt crack.

we are not sure what it is, he was in a hottub yesterday that uses bromine as a chemical, he was also playing outside in some tall grass and weeds, it was quite warm and muggy out yesterday as well.

Any thoughts?

Jeremy