E.T.
NONE.
According to the Dr. Sears website, the dosage is basically 0.5 mg per pound. So at 21 pounds, it would be about 10 mg. But Childrens Benadryl is not recommended for kids under 2. And it contains a sedative... which can be seriously dangerous for kids under 1.
What do you think he needs this medicine for? Perhaps we can help you find something safer.