Hey there we're so glad you posted. Sorry to hear about Finley's issues right now.  By any chance has he seen a canine rehab therapist yet? If not, that's a great place to go while you figure out the best ways to manage his arthritis.

To answer your question about harnesses, yeah, it can be challenging to find a good harness for a front-leg amputee. Without that other leg to keep harnesses in place, they do tend to rotate. The Ruffwear Webmaster harness rotates the least of all, but for dogs with limited mobility as you're describing we recommend the Custom Pet Support Suit. It costs more to have one made to Finley's measurements, but it will fit better than an off-the-shelf harness will and it's a good tool to help you support him. 

If it's his rear-end strength that's wearing out more than the rest of him, you may need a harness with rear leg loops. We love the Doubleback by Ruffwear but it's not for everyone. You can click on the link for a review. The new Care Lift Harness is getting good reviews and may be more of what you're looking for. 

Let us know which one you try and what works for him.