Hello,
Sorry to hear about your problem with your puppy.
This is very unusual for a neutered dog to be in this much discomfort after surgery and i am suprized at your veterinary surgeons lack of concern.
I would definatley not give him anymore asprin as this can be toxic in dogs , they can not metabolise it like we do and can have effects on the kidneys.
There is definatley a reason for your puppy to be in pain and he should be examined by the VS ASAP.
It could be due to a number of reasons -
1: Puppys can be very vocal when it comes to pain and castration is a very big operation, however if your puppy is in pain and it is getting worse then pain relief should be supplied by your veterinary surgeon
(sometimes though this can mask the problem)
2: The top layer of skin can become very itchy approx 3 -4 days after surgery , this is normal , his scooting could be a sign that it is itchy and your puppy may well be frustrated that he cannot satisfy this.
3: You decribe his back being hunched , this again is a classic sign of pain and there is no reason why pain relief should not have been supplied.
4: The internal sutures may have ruptured causing a small hernia , but with this there would be a small swelling and inflammation around the incision.
5: Have you or the vets checked his anal glands - anal glands are two tiny sacs that lie either side of the rectum and when the dog passes faeces these empty. In a dog that has just had surgery the mixture of drugs used can initially slow the digestive system down causing the sacs to fill up (as they are not being expressed by the faeces, or simulary if he has had dirrohea) If the anal glands are full it can cause irratation , and pain. One of the classic signs are them scooting along the floor trying to relieve this problem - please get theses glands checked it doesnt cost much , theses can be expressed by hand externally or internally by the veterinary surgeon or even yourself if you learn how to.
I would make a appointment to rule these possibilitys out - even if theses are ruled out at least it puts your mind a rest and your puppy can have a post neuter check (most practices do this anyway at day 4 -5 routinley free of charge)
At the least you should come away with some pain relief probally being either Rimadly tablets or Metacam liquid.
It is not fair and not acceptable that he should be experiencing pain 4 days after surgery, and if we were to have the same operation we certainley would not be discharged without pain relief!
I do believe that you should return to your local vets and have him checked out.
Please let me know how you get on . Good luck . xx