Cough

Hi all! I have a 6yo who comes down with a cold about every three weeks. I am new to homeopathy and am having a hard time finding a remedy. What happens is he gets run down refuses to nap on the weekend and the by Thursday (like clockwork) he is sick. Usually, it starts with a fever then turns to a dry barking cough. This time he did not get the fever just the cough. His cough is a barking, dry, croupy sounding cough. Its constant all day only feeling better by being outside. He feels better drinking warm liquids. He complains of a burning in his chest that he associates with when he has had a sour taste in his mouth. Not that its sour when he coughs just the burning feels like when he has had a sour taste in his mount. This has been happening every 3 weeks since he started school in August. Im looking for a remedy for his cough as well as something to give him when he is run down to prevent his from getting the cough. Any help would be appreciated! Thank you!