What do you suggest to strengthen a dog's mind to overcome his fear of thunder?

I don’t believe it’s always possible to get dogs over this. I think it’s a natural response for many animals to be worried about thunderstorms. Some dogs with weak nerves show this, but I have seen some really solid dogs exhibit a fear of thunderstorms as they get older.

I have heard that some people have used products like Rescue Remedy (usually available at a health food store) or Melatonin to help dogs learn to relax during things like this.

I think it’s best to have a safe and comfortable place for a dog to go if they appear anxious, I don’t believe correcting a dog for anxiety is going to do anything except make it worse. If you get upset or annoyed at the dog for showing anxiety it will make the dog MORE anxious so try to be neutral if you can.

You could try doing something the dog really loves to do when a storm is approaching and see if you can put a positive association with it, other than that I know this is something that can be a real pain to deal with. One of my old dogs got quite nervous about storms in her later years, and I never could get her completely comfortable once she started exhibiting the worried behavior.