There are many possible reasons for this, but your dog should be taken to a vet immediately. One possibility is that your dog is having an allergic reaction to something, which caused his eyes to swell up.
Possibly he has an eye infection. Check for discharge or mattering. He may also have pink eye. If condition persist a visit to your vet is important so he does not get permanent damage
This could be two things. One, your dog is very tired. Did he have a change in food or medication that is making him sleepy? Two, the dogs eyes are irrated and it hurts to keep them open, are they inflammed?