Call your son's pediatrician to confirm this, but most nurses will ask you:
1. What other symptoms does your son have, if any?
2. Any change in mood or behavior?
3. How old is your son?
4. Tell them more about the bumps. They will want to know if they are itchy or painful. They will want to know if they blanch. You can tell this by pressing lightly on a bump (as long as it does not cause pain), and if the color of the bite turns to skin color or white ("blanches"), it is typically not systemic (meaning: only on the surface of the skin). Blanching is a good thing, but, if the bumps do not blanch, this is OK...it could be a rash caused by a virus (many childhood viruses cause rashes, and the body heals itself).
Let us know what your son's pediatrician (or nurse advice line) says, and keep us updated!