What to do about Neighbor’s Dog??

Hey, everyone!! Sort of a long story but I’m 19, living with my parents, and our next door neighbors have a boxer. We’ve had a great relationship with these neighbors, and they’ve been nice over the years, but it seems like they neglect their dog. He’s not even two years old, and they just leave him outside on their raised deck. He doesn’t have any form of shelter, or anywhere to go, he just stays on the deck 24/7, even at night. They don’t bring him inside during rain, thunderstorms, snow, or anything. This wouldn’t bother me as much if he had a little dog house at least, but he doesn’t, he’s just uncovered. I’ve met this dog once or twice, and he is the sweetest thing I’ve ever seen, just super nice. My mom and I are unsure of how to approach the situation, since we have three large dogs, we wouldn’t have much room to take in another. Also, we don’t want to seem like we’re insulting our neighbors. We are getting our back yard fenced in soon, and we were thinking that we could offer to keep him in our back yard for them, meaning he would probably end up being ours eventually. Any tips or ideas on how to save this baby sooner?? We have such bad rain right now and I hate seeing him suffer out there in the cold every day.