After taking wellbutrin xl 150 mg. for 2 weeks, my 13 year old so...

After taking wellbutrin xl 150 mg. for 2 weeks, my 13 year old son broke out in large red hives that over the course of the last few days have covered his body from head to toe. He has joint pain and is itching like crazy. Doctor prescribed prednisone and he's also taking benadryl. Doctor thought the hives could be a virus but doesn't seem so given all of these posts. He's stopped taking the wellbutrin today. The wellbutrin was so helpful for his depression but the hives are horrible. Any idea how long the hives will last?