How much benadryl (diphenhydramine) can my 11 year old son have? he has a red rash over most of his body. please advise.

25-50 mg and SEE DOC: Assuming average weight of about 80lbs or 35kg, you can give 1-2mg Benadryl (diphenhydramine) per 1kg of weight = 35-70mg. Variables would be his weight and the severity of the reaction, as well as how often you plan to give it. A one-time, 50mg dose would be appropriate, or 25mg every 6 hours till symptoms improve. Also consider time of day and drowsiness side effect. You can safely give 200-300mg/day if needed.