He is dreaming. You'll find he also paddles his feet as if running. This is normal.
That doesn't sound normal. If he is not conscious, and you can't get him to respond when it happens, my guess is that this is a seizure. Ask your vet to do a full neurologic exam. At this age, sudden onset of seizures or neurologic signs like this could indicate a brain tumor.
Have you noticed any other problems lately with walking or eating? Blindness, deafness? Is his face symmetrical or does one side droop? Is he circling to one side when he walks or is there a head tilt?
I'd say absolutely bring him to see the vet. It sounds like a partial seizure. Your vet should check his cranial nerves and do a neuro exam on his feet and legs to check for something called proprioception. Some vets can give you a script for valium to give rectally at home when seizures happen to break the cycle. Ask about this. Other than that, your dog might benefit from anti-convulsant therapy such as Phenobarbital. They are sleepy on it for a while, but then they get a tolerance for it and behave normally. It will hopefully suppress these episodes, which as you describe them seem quite disturbing.
I have to apologize for my first answer. We do get many questions about the dreaming/yelping. Please take no offense by my initial interpretation of your question. I shouldn't have assumed (you know what they say about assuming), as this is genuinely a very serioius concern.