He was very active throughout the sonogram, and he is perfectly healthy! It's neat to see how much he is moving, since at this stage I am only feeling him every few days. The doctor said it will be a few more weeks before I should feel him every day. He is about 5 1/2 - 6 inches rump to crown, and he weighs ~8 oz. Here are a few pictures.
His profile with his left hand touching his forehead.
A leg.
His left hand.
His foot.
Proof that he's a boy.