A cough can be several things.
Worms, pneumonia or too dry straw.
Was he wormed when you got him at 8wks old, if not i would worm him for good measure, won't do any harm might do him some good.
2nd check his temperature (should be 38.8c or 102f) and general demeanour, is he eating? lethargic? High temp suggest pneumonia needs antibiotics on a
NOW basis.
3rd check his throat for obstructions and rub under his chin down his neck, could just be dry straw or something gone down the wrong way.
Check his siblings - coffs & sneezes spreads dizeezes!
I am not a vet and can only offer advice on the info you give, if he is in any pain or duress get your vet to him asap which is what i'm sure you would do.
HTH
mandy