Can An Itchy Abdominal Rash With Lumps And Finger Swelling Be Treated?

My seven-year-old son has for the past two days developed a rash in the evening, after we get home from school. It appears, before bedtime, as an itchy red rash on his abdomen, with very small raised bumps - smaller than mosquito bites, and as a raised rash with very large bumps on his ankles. Yesterday during the school day he also complained that his hand was itcy, and his fingers were swollen. The rash and itch have been gone each morning. He has not had a fever or other symptom, with the exception of a vague complaint yesterday that he did not feel well during the day.