You can’t really OD on Vit C. When my guys were sick I was giving them 250mg 2 times a day. If that doesn’t upset his stomach you can give a little more. Ester C is good too because it releases into their systems a little slower. Also for kennel Cough you can give Robitussin-DM to give him some comfort. And try adding some healthy stuff to his food like egg, plain yogurt, fruit & veggies (if he’ll eat them) because that will help boost his immune system too.