Re: One breast "snacks" at night?
Welcome to the forum! I am sure you're exhausted. But what you describe sounds pretty normal for such a young baby. Here are some possible explanations for the behavior you're seeing:
- reverse cycling: http://kellymom.com/bf/normal/reverse-cycling/
- reflux (baby may be uncomfortable lying down)
- baby misses you- isn't that sweet? 
I honestly doubt that the issue is that baby isn't taking in enough milk from his single-breast feedings. If he sometimes does this during the day, it's probably even more likely that he'll do it at might since most moms make more milk at night. It might take both sides to satisfy him during certain stretches of the day, but only one in the wee hours. My MIL, who is a nurse-midwife and longtime breastfeeding activist says that this normal production pattern proves that babies are meant to be nighttime feeders. Which is not a consolation!
Coolest thing my big girl said recently: "How can you tell the world is moving when you are standing on it?"
Coolest thing my little girl sang recently: "I love dat one-two pupples!"