My neighbors are OK with the idea of roosters, because our kids are in 4H. Our DD has done poultry projects for the past 6 years and now DS is getting started. Although the neighbors are great and very supportive, I want to be polite. My orpingtons do not crow much during the day, so I keep the 2 boys in the garage at night. When they crow away in the morning, it's muffled & barely heard. I made a short box which prevents them from stretching up to belt out their loudest crows. They can stand & walk - but with their head down. It's only for the night, so not a big deal. They also wear crow collars which limits & controls the volume. There's no way to stop them from crowing, but there are some ways to manage it.
After you try a few things, talk to your neighbor & ask if the noise situation is better. Just the fact that you listened and tried to make things better will go a long way. If the crowing still bothers him, you may want to seriously get rid of the roosters. As much as I love our chickens, good relationships with the people around us on whom we rely is more important. I always ask them to let me know personally if something is bothering them. I also bring them fresh eggs, backyard chicken soup, garden veggies, & compost for their gardens and happily let them throw their kitchen scraps over the fence. When we started the flock, building the coop was sort of a fun neighborhood project. That's why I welcome their help. When the neighbors have grandkids visit, our backyard is the fav spot to visit.