My new neutered puppy is lethargic and won't walk. Is he in pain?

I am very sorry to hear rhay Spock is not acting very energetic. I believe it could be because one of the following reasons;

Spock could still be adjusting to the change in his life. The last couple of days have involved a surgery and a new home and family. All of this may take time for him to adjust to. He might feel slightly insecure and could benefit from some of the following techniques to help him adjust;
Extra walks
Extra interaction time - petting, and games
Special meal of boneless boiled chicken breast and white rice

It is possible that Spock is still in pain or that the surgery site is not healing well. Look for swelling, redness, heat, pain or discharge in the area. That would indicate infection. If you notice any of those signs, you believe that Spock is uncomfortable or not healing well take him into a veterinarian for a check-up. It's usually a good idea to have a new family pet addition examined by a veterinarian within the first month anyway.

Best of luck with Spock!