Blood work must be checked on any dog to ensure they can metabolize medications prescribed.
Was blood work checked on your dog and NSAID therapy ruled out?
NSAIDs- non steroidal anti inflammatories like Rimadyl,Metacam and Dermaxx work very well in healthy dogs.
They ae safe forr long term use if blood work is monitored afte 2 week then every 3 months to ensure organ function is adequate.
Joint supplements should always be administered with omega fatty acids like 3V Caps,Derm Caps or Omega Plus.
These acids disrupt the inflammatoy pathway decreasing the amount of discomfort that is perceived.
Besides being an anti inflammatoy omega fatty acids are great fo the heart,skin,coat,joints,kidneys and brain.
How could you go wrong!

Supplementation with glucosamine/chondrotin/MSM and omega fatty acids enhance the effects of NSAIDs and othe pain management.
When using supplements and NSAID and pain is still not managed the vet can prescribe tramadol to help ease pain.

Dont give up on managing you pets pain.
Quality of life is impotant and medications are available to help.
Best of luck

Add:
Adequan is excellent.
Sounds like you pretty proactive treating him.

I think additional supplementaion will benefit the dog.
Especially adding in the SynoviG3 which offers the MSM.
Also omega fatty acids will complement the NSAIDS,Adequan and any additional supplements.
Honestly if I didn’t see the vast improvement in many animals with the use of SynoviG3/Arthoplex I would not recommend it.
I saw such a difference in these animals I started supplementation myself which human versions.
I have looked into it and the animal data on glucosamine/chondrotin supplementation is much like the human reseach.
Much of the research shows little benefit in using the glucosamine/chondrotin in animals just like humans but II have seen dogs near euthaniasia get a couple of years or a skip back in their step with Synovi.

With the previcox you want to watch fo any GI upset or straining.
If you pooch is shandling it well and it is helping that is great.
GI effects can be a problem so anytime the dog has an issue do not over look it.
Have blood work checked as he is older and has a chronic illness.
I would ask your vet if they have ever prescibed tramadol/Ultram in conjunction with arthitis treatment.
Tramadol is great as it doesn’t have the nasty side effects that many opioid and NSAID medications do.
Its an opioid like pain reliever.
It is one chemical process away fom being an opioid so it does offer relief.
The only downside is I have heard some dogs get grouchy.
Sp it may affect their temperment.
I would certainly discuss Tramadol with you vet as we prescibe it often for pain management.

Hope this helps!

The vet thinks he could have disc trouble but it could also be tumour or spine fusion. He is 11 at least. (rescue).

He is panting terribly on it and drinking and peeing loads. But for my old boy it is a last resort. Haven’t got any good results yet.

As for the arthritis, then other than adding a good glucosamine or greenlip mussel or shark cartiledge to work alongside the NSAID’s then I can’t suggest anything else. Have found these to be beneficial.

I also have had amazing results from accupuncture, and have used it for various things. Only problem is that they will not do it if there is cancer in the system, as it accellerates it. There is the risk at 15 that this could be so.

Would still give it a try if it were me though, as recommended above.