What Causes A Fluid Filled Blisters On The Legs?

My 9 year old son, who weighs 120lbs, had developed a quarter sized clear fluid filled blister saturday, the MD said it was an infected bug bite and he was placed ATB. He has since developed 9 more on both legs, and his left arm. He has had 4 doses of ATB. He is afebrile, has been c/o leg pain (Muscular) for the past couple weeks. Has HX of Asthma, on Singulair and flovent. HX of FX Lower L arm with ORIF at age 6, Toncilectomy for hypertrophy of toncils causing airway blockage. NKDA. Should he be reseen, or should we wait longer?