Hello, I'm sorry to hear about your dog.
It sounds like there is definitely something going on here that needs to be addressed. The roundworms are more likely due to an underlying immunosuppression rather than a primary problem. Roundworm infections are not all that common in older dogs as they can mount a response and clear them without medication. The fact that this 8 year old dog did not clear a roundworm infection without meds is a big red flag that the immune system is not functioning properly. Now we need to find out why.
I would start by running full bloodwork on him. This will test organ function, electrolytes, calcium: phosphorous balance, white cell and red cell counts. I would imagine that this will help narrow down the long differential list. More specific tests for endocrine function may be needed if Addison's disease is suspected. This is where the adrenal glands do not function properly.
The spinal pain is a little odd and may be an incidental finding here. Unless there is cancer that has spread to the spine, it's hard to tie in neck pain with immune system suppression and the rest of the GI signs. Doing a good neuro exam will help determine if the spinal pain is actually neurologic or if it's orthopedic in origin.
Obviously there is quite a bit that needs to be done here, but a good thorough exam as well as screening bloodwork should narrow the list down and hopefully diagnose something that can be treated or at least managed to make your dog more comfortable and get him back on the road to recovery.
I hope this helps, let me know if you have any other questions.