I'm certainly not a vet and times may have changed but I had an old dog that for the last 3 years of his life was on a daily regimen of 100mg aspirin a day per vets orders.
You don't say how old your dog is or what kind of pain he has. I think I'd see a vet before medicating him. If he just has old age related aches and pains the vet may very well recommend some kind of mild pain killer. If he's not that old something is obviously amiss.
Never give medication to a dog before knowing what you're doing and what you're giving at what dose. If it were a different pain med, you could have killed your dog.
Please take your dog to the vet. This does not sound normal and you could do a whole lot of harm trying to treat your dog at home. He could be arthritic, dysplastic, he could have spondylosis, DM, a back injury, an impacted anal sac, a hotspot, an internal injury/issue, who knows. Your vet needs to see the dog BEFORE you give any meds to better make an accurate diagnosis.
Just got back from the vet today. Duff has arthritis it not too severe yet but we are giving him supplements as well as some prescribed pain killers. Hopefully these will help diminish the pain and slow down the arthritis. Thanks for all help and comments.