Hi, I can help you with your question today.
It's difficult to say without seeing the action, but my first suspicion would be that he is dreaming.
When a dog (or human) dreams, the descending motor pathways should be blocked, preventing movement during REM sleep.
In children and puppies, these neural pathways are not always fully blocked.
That is why more children sleepwalk than adults, for example.
I suspect this is the type of movement you are seeing with your puppy.
Slight motion that appears as shivering due to minimally engaged motor pathways.
Since he is otherwise active and eating well, I do not think that this warrants a visit to your regular vet.
Please let me know if you have further questions. I am happy to continue until I provide the service that you seek.