He doesn't like it raining cats and dogs

Cooper does not like the rain. He avoids it unless it's absolutely necessary to get in it, and rarely even ventures outside. In fact, I typically have accidents in my home during a storm, simply because he won't even go outside to potty. This is problematic for me, in every way, but since he's scared, he kinda gets away with it. I just clean it up and we deal.
Today though, I decided to start working him through his rain issues. It was 5 p.m., walking time, and the rain was still falling. He didn't seem to put two and two together as he got all exited about the walk, but that's the fun of having a puppy. Once he was leashed up, and we exited the house, he stopped, and made a quick turnaround. I did not allow it though, and we proceeded on our walk ... in the rain.
He rushed through it, certainly, but also made all the stops he was supposed to, including #1 and #2. I brought him home, and toweled him off so he could have fun with that. He got his treat. I told him many times what a good boy he was. So far, for the rest of the evening, I've had no issues inside the home. I'm hoping that when I wake up tomorrow morning I can say the same. He's the first dog I've had that makes me so pleased that we're in the desert, where there is very little rain. Still, he needs to get used to it, and I'll continue to push the issue until it isn't an issue.
Also, I took Oliver and his gimpy knee for a very short walk up to the mailboxes. It took us about 20 minutes because he moved so slow, but that didn't matter at all. He was so happy to be out of the house. I can't help but think that a little exercise would be good for him at this point. I worried, of course, that I was pushing him, but he seemed to do fine for the rest of the night. Which reminds me, he needs his nighttime pain pill right now.