Suggest Cure For Skin Rash

my 13 year old has a big raised blitvhy rash on one side of his body on his tummy and thigh and arm. he had a severe hedache when the rash came he had the rash fir three days but has only just told me. his firehead was very hotast night and his eyes looked quite red. hr was playing in bushes for last three days so my hubby thinks its a thistle alkergy?