What Causes Tiny Water Blisters On The Leg With Spider Bite?

A couple of months ago my son had a spider bite (doctor s assumption...we didn t see the spider). His body wasn t healing the inflammation from the bite properly so he was put on an oral and topical antibiotic which healed the spider bite (although, still redish/purplish but now smaller and flat--scar?). However, around the same time he has what appears like small water blisters appear on the same leg. They are still there despite the spider bite disappearing. They are not red, not itchy and not painful. They are about the size of an eraser on a pencil and appear to be little, clear blisters. Is this a cause for concern? Treatment?