Thanks again for your question. The problem you report is difficult to treat given that the area were the pain is located is always exposed to further pressure any time your father sits down.
If the pain is chronic in nature, you may want to consider a visit to an allopathic doctor (MD) with the goal of obtaining a prescription for gabapentin
given that a this instance and given that the pain has been present for ten years, there may be some nerve inflammation
Other options include a trial injection with an steroid
in the area, but this also requires the intervention of an experienced pain clinic provider.
In the mean time, relieving pressure from the site using a inflable donut may help some. I hope this helps.