Cankers are very painful for horses. I have a friend who had to let her fellow go to the other side because of how bad his cankers were. They even did surgery and they came back - it was so painful for him, he didn't want to stand anymore.
I'm terribly sorry that your darling is going through this. I have not had to go through this, so I cannot give that much advice, but I did find some links for you on the internet that may beable to help you. Equine Podiatry | Dr. Stephen O'Grady, veterinarians, farriers, books, articles NaturalHoofcare/thrush/canker in horses.html
If this were my horse, whom is a 21 year old TB turning 22, I would do all that I can, to make sure he remains healthy and happy. I would get as many opinions as I could on the case and take the needed steps to ensure his happiness.
I wish you and your beloved all the best! Keep us posted!