A lot of people - and a lot of vets - rate daily Glucosamine for older dogs to help against arthritis. I certainly noticed a difference in my two old boys.
If you do find a coat, or anything else that helps, please report back. I have the same with Skip, he loves getting towelled down but it's not a perfect solution. And this winter I shall be wondering which is best - take him with me to hang around while I pick horse poo, or leave him behind and just take the more active and weatherproof Dot.