My son has a really sore throat and a cough. I got him Robitussin Cough Gels for the cough. My problem is it says for 12 years and older take 2, he is 12 years old, but although he's in the average range for his height and age, it's low average. He weighs 80 lbs. is it safe for him to take 2 or should I give him 1. The one I gave him seems to have helped but not completely, and the more he coughs the worse his throat gets.