if the swelling has gone down thats a pretty good sign that this time it was just a ligament strain. i'm guessing he landed wrong or on someone's foot and 'rolled' the ankle? the healing process of damaged tissue can leave it weaker than its original strength, so therefore he is going to be susceptible to reinjurying that ankle once again.
Yousuf mentioned a great way to prevent them is by recognizing early when you've landed wrong and just crumpling, taking the weight off that ankle. and i'm guilty of this, but make sure he is not using low-top (e.g. running) shoes to play. basketball shoes are high-tops precisely because ankle injuries are one of the most common in the sport (along with bruised egos and shattered dreams of becoming the next mj)
i'm getting there.....just gotta grow another 6-12 inches