This is so very sad
There is no winning here for this poor old pup as your aunt and uncle are not receptive to advice and have obviously become immune to the pain and suffering their pup is in.
If you report to the SPCA, and they do indeed take the pup, he will likely not be a candidate for adoption due to the aggression you mentioned.
I would continue to appeal to your aunt and uncle but try to be more specific with regards to how much pain their pup is in. Compare pups rotting teeth to a rotting tooth your aunt might have had and how much better she felt after her dental visit? Perhaps offer to help them clip pups toenails and talk about how much an ingrown toenail hurts every step he takes? Talk about how old pup is and how he should have an easy life as an elderly pup and not suffer pain each and every day?
These folks are in total denial and need someone to LEAD them into better behaviour towards their pup. Hopefully that someone will be you.