Benadryl for 20 month old?

My sons allergies flared up really bad today and I can’t get him into the dr tell Monday. I looked up how much Benadryl to give him and what I’ve read says, the dosage is based on weight, but age. But kids 2 and under shouldn’t take it unless advised by a doctor. But I’m confused because his doctor has prescribed him Benadryl before around spring time when his allergies got really bad. He’s 30 lbs, so he’s plenty big enough according to google just not old enough?