Make sure he stays hydrated. Young puppies can become dehydrated quite quickly, although it's more of an issue when they have diarrhoea or vomiting than with a cold. If you think he isn't drinking as much as normal you could try poaching a chicken breast in water (don't add salt or anything) then encouraging him to have little drinks of the cooled poaching water.
If he seems quite well in himself, is eating fairly normally and isn't shivering or having breathing problems, then it's probably just a cold and he should get over it quite soon. Just be a bit extra careful as he's so young. If you have any doubts phone the vet for advice.
Hope he's back to normal very soon