Take his temperature before you speak to the vet, also did he get colostrum and was his dam vaccinated with Heptavac (or similar clostridial vaccine containing stuff for pasteurella)?
If no colostrum and/or un-vaccinated dam - he will need a vaccine shot soon, but you will need to sort out his cough first.
It could be dusty hay (if that's what he is mostly on). If it has been going on for a while it is unlikely to be pneumonia, as he would be dead by now. But you need to rule it out.
Lungworm is another possibility, so get that checked out.