My 4 wk old pup has a runny nose!?

hi i have a litter of king charles x collie pups they were all good sizes but the biggest one has now turned into the runt,hes 4 wks old and tiny compared to the rest hes eating/drinking fine but does look lathargic and small,and hes got a bit of a runny nose,no others have a runny nose just him.