What Causes Itchy Spots On The Lower Body?

In the last 24 hours my son has developed welt-like spots on his abdomen and legs. They are different sizes - some 5 cent piece, others a bit larger than a 10 cent piece. They are itchy. At bath time last night there was only a few of them and none were large or raised, this morning they are all over his belly and causing him to itch. My son is 2 years old, no historical medical issues to speak of.