My 5 1/2 Year Old Grandson, 49 Lbs, Was Diagnosed

My 5 1/2 year old grandson, 49 lbs, was diagnosed with Strep Throat today and started Amoxicillin. No previous allergy to amoxicillian. He also had the beginnings of a rash BEFORE the amoxicillin, nurse said rash probably result of strep. He had the amoxicillin at 7 pm, played, ate dinner, and for the last hour has been itching like mad, can't sleep, whining for the itching to stop. Can I give 1/2 tablet of Benadryl?
I don't have any children's antihistimine on hand.