My bridge bunny Roger had a stroke and was left paralysed down his right side, and struggled to get about because of his weak back legs, but I saw a programme on the tele about 'Special Needs' pets, and one of them was a rabbit who had a cart made to fit him by 'The Dog Mobile Company'. Contact number is 01262 851632. They are an absolutely brilliant husband and wife team. Angela takes the orders and will discuss your needs fully with you, and then Bob makes each item individually to fit the pet it is required for.
I didn't have a cart for Roger, but they had an excellent product called a 'Quicklift' which is what I bought for only about £22. Basically it is like a sling to go underneath your pet, and you are then able to walk along the side of your pet, supporting him, but allowing him to actually get his legs working and build the muscles back up. I really can't praise it enough, and the more time you are able to put in, the quicker you will see results.
Initially, I started off just doing a couple of laps in the front room, but within about 2 weeks, we were actually walking outside in the garden, and Roger really perked up becoming mobile again. It helped him a lot, but because of being paralysed one side, he wasn't able to get about himself unaided, but the health benefits in getting him moving again were tremendous, and he was able to move about on his duvet by himself.
If you decided to try this, you may well actually find as Berry is not paralysed, that it does then get him strong enough to get about himself. I don't actually see any reason why it shouldn't if it is simply muscle wastage, as really all you need to do, is get those legs working again.
I'm not so sure if massage is of too much use, but when Roger was sat up with me, I used to do exercises with his back legs, such as fully extending them very slowly and gently and with your free hand, apply some resistance to the feet so that they have to work harder to fully extend. It's best to gently bring one leg at a time as close to the abdomen as possible, and then fully extend, applying some resistance. Another one was doing full circles with the legs. Basically, you just need to get the legs working again, and the more you work them, the quicker they become stronger.