What Causes Bump And Itching Between Fingers And On Buttocks?

Hi, my 3-year old son has had several (not sure if related symptoms): one blister-like bump on his finger for 2 days - no complaints; itching between two other fingers - no visible sore - 1 day; and I just noticed pimple-like bumps on him buttocks! (no complaints either). No fever. Runny nose, though. I am really puzzled - never had anything like that before. Although he does have eczema that seems to be food-related. Thanks