Red bumps in groups - become scabs

Hi,
My son has been getting red bumps on his body in groups. They usually show up in the morning and are very itchy. Over the next couple of days they become hard and almost seem as if they have scabs. After about 10 days they go away. We've been to the dermatologist and they have many theories but are basically not sure. I wonder if we have bugs (bed bugs) that are biting him over night and that he is allergic to the bites. No one else in the house has them, and neither does his brother who sleeps in the bunk above him.
Does anybody have any ideas?
Thanks