I use the regular Children's Dimetapp (not the infant formula, which has been pulled off the shelf) and have done so since my son was 3 (he is now almost 7.) I am pretty sure my doctor told me how much to give him then and also my daughter who would have been 1 at the time. PLEASE ask your doctor though, because the label starts dosing at six years old!
I prefer the Dimetapp over the Benadryl for 2 reasons: 1 is that the taste is better (my son won't take anything but Dimetapp actually, which is extremely annoying) and 2-it has an antihistimine and a decongestant and usually they need both things.
BUT...I rarely give my kids cold meds at all! I will only give it to them at night IF they are coughing from the drainage. Otherwise I do not give it to them. Yes, it's gross that their noses run, and yes, I did, when they were little, have to run around wiping them till they learned it for themselves! However, my youngest has a heart defect and when I was discussing cold meds with the cardiologist, he said that it is best to not give ANY child cold meds if you can avoid it because it does affect the heart rate of some children.
So I really do have to agree with the pharmacist who responded...the saline is good if he is congested or just let it run it's course. Especially during the day. I have used the benadryl with my daughter at night for the cough and it does work, especially if the runny nose is just clear running sort of like a faucet. Good luck!