He's probably ready to move to his adult home. A 20G long is good if you are going for aquarium housing. You could get something bigger if you want a great display, as long as it has at least as much floor area as a 20G long. Babies get stressed by too much space. I think it's because in the wild when they are little they are food for almost everything, but at 1 year and good length on him he won't be stressed by moving straight into adult quarters.
Edit: Why buy more vivariums (viraria?) than you have to, when he can move into his adult home and then finish growing up? Unless, of course, you want to have a spare around, then you could move him into a tub home for the next year or so then and save up for a fancy final home for him. The spare would then be ready to be cleaned and used for ANOTHER snake, and you know you NEED another snake, right? Oh, and when he moves you can get a 2010 hatchling & put them in the 10G, too.