G.B.
I would not follow that docs instructions. I would talk to a Pharmacist. However, they are going to tell you that Benadryl is not made for children below a certain age.
Children have died from taking meds that are for older children, they only took a reduced amount but they died because it was not formulated for younger kiddos. Find an alternative. There are many things you can do that will alleviate the symptoms with out the drastic side effects of Benadryl.
I gave my little guy some Claritin one time, only half a dose and it dried him out so bad his nose bled. I shudder to think what would have happened if I gave him a full dose.