Unless he's in severe pain I would never ever have him put down. Our dogs have always been our babies so I am totally with you. Here's hoping with the right diet you will have him around for awhile.
As for the diet, I'm not experienced in one for renal failure, but I am dealing with kidney stones in our dog. We have a Dalmatian that's a stone-former (he had a complete blockage when he was only a year old and needed surgery). To clarify, Dalmatians typically get urate stones, which are a bit different than the calcium stones that most dogs form, so this might not completely apply to your dog (or to the renal failure instead of just stones). I just figure any information I can give might help. But definitely check with your vet first!
Anyway, after the surgery on our dog, the first thing our vet recommended was prescription U/D food, which is very low in protein. After a few years on the prescription food we didn't have any stone recurrences, but our dog gained a lot of weight and his skin and coat were terrible. The vet just said he was getting too much to eat, but even cutting down on the food didn't help. I started searching for other food options, and ran across
The Honest Kitchen. All their foods are human grade and natural without all the chemicals you'll see if you look at the bag of prescription food. It's dehydrated, so you just add water and let it soak for at least 10 minutes before giving it to your dog. Since you add water to the food, I always add more than required and make a sort of soup for our dog so that he gets lots of extra water (good for flushing the kidneys).
Since he's been on the Honest Kitchen food, he's lost 10 pounds (now normal weight), his skin is no longer flaky, and his coat is so soft. I can't believe what a difference the food made. The vet has also been testing his urine for us, and it's been about 6 months and he's still good.
I would consider calling the Honest Kitchen folks (I called them for more information after researching their stuff and they're absolutely great) and see if they are familiar with using their food for dogs with renal failure. They won't try to sell you if they think it could be bad for your dog at all, but if they have any experience with their products and renal failure they will recommend the best choice.
The other thing we changed with our dog was his water. We have a well, and sometimes well water (due to all the minerals in it) can be a contributer to stones. We now only give our dog distilled water. I figure every little bit helps. Do we probably spend too much money on food and bottled water for him? Yep. But as I mentioned before, he's our "only child" and as long as it's not putting us in the poor house we do whatever it takes.
Hopefully this will help at least a little, and I wish you all the best with your boxer!
I'll warn you the "presentation" isn't the best- it looks like grass soup once it's mixed up, but he doesn't care one bit. 
He's got it figured out now though, and he keeps his head in the bowl. Although I usually catch him for a mouth wipe afterward so he doesn't wipe it on our pants instead. 
He also knows the drill, and as soon as I mix it up he'll sit staring at the counter until it's ready.
