rash on toddler's butt

My son is just a little over 2 1/2.  Over the past week or so he has had a strange rash on his butt. On some places it is small red bumps that look like bug bites, then on other spots it is much smaller red bumps that spread out over a larger area. It seems to be itchy only at night before bed and then in the morning it appears to be mostly gone and looks like it's healing. Then as the day progresses it gets itchy and inflamed again. I thought it may be poison oak but it doesn't seem that way now, because it never weeps or oozes. He doesn't behave any differently and has no symptoms other than it being itchy. When the bumps first appear they have a whitish center. Then that goes away and it is just red. Im not too terribly concerned because he doesn't seem to be sick but I would like to figure out what it is so maybe I can prevent this from happening again in the future.
Let me know if you need anymore information on this and thank you in advance.