Hello and Welcome!
It's so hard as they age. I know from experience as we home senior labs. First thing, a good checkup with a senior blood panel to see what is going on. Make sure there is no urinary tract infection.
Our Melody was quite unsteady on her back legs due to lower spine arthritis. She was also fecal incontinent due to this. After a good check, she was put on Tramadol and it helped her tremendously. The fecal incontinence stopped.
We also have rubber backed rugs all over the house for better traction, and this includes the back deck. Also, DH build a ramp off the back deck, even though it was only three steps. Ok, they are not fashion statements but I could give a flip; it gives the pups traction. All of ours have have ortho beds for comfort. Got a walking harness from Ruffwear Ruffwear Web Master Harness | Web Master Harness | WebMaster Dog Harness | Ruffwear Web Master Dog Harness | JJ Dog. You notice that there is a strap well back under the belly. The handle on top is strong and rigid. It may look complicated, but it's easy to get on and off, very adjustable, and all buckles and adjustment areas are padded. I could give her the extra support she needed when going potty. There are other harnesses on the market that look just as good.
Another thing I've done with all our seniors is range of motion exercises. With your boy laying on his side, place your hand gently on his shoulder and move his leg slowly and gently forward and back. It takes a little practice and your boy getting used to, but it does help. I would do each joint in each leg, gently and slowly. At first, he might resist. Do what he tolerates. It has become a ritual here, after dishes done in the evening it's massage time and my guys look forward to it. I talk to them throughout the routine in a soothing voice; it turns out to be a precious time of loving and helping in any way I can to help them be more comfortable.
Sending lots of mojo (very powerful lab board stuff). I know how you feel. Please don't hesitate to ask questions on the board or PM me if you need. I think you need 5-10 posts to start using the PM feature.