How old is the swimmer in question? If he is younger than 14, then his problem most likely has nothing to do with muscle mass. The amount of muscle on a swimmer under 14 is insignificant in comparison to technique. Yes, some younger swimmers have more muscle than others, but it will not make that much of a difference.

Most likely, your son is not catching any water when he pulls. When his arm pulls under the water, he is probably pulling with a straight arm, and there is little or no resistance there, and he is not pulling himself through the water. This would explain the fast rate at which he moves his arms. Under the water on his pull, it is important that he bend his elbow slightly as he pulls down through the water. He will feel the resistance of the water and it will be a more tiring and more difficult stroke at first, but it will pull him forward more and faster. On the back end of his underwater pull, he needs to finish his stroke. It is important to pull all the way through the stroke, and follow through as your hand exits the water near your thigh. If he is lifting his elbow first, and his hand is being pulled out of the stroke early (around his waist), then he will not gain the full advantage of his pull. Changing his stroke to get a stronger pull will lokely slow down his stroke rate, but it will make him a faster swimmer.

Another thing to consider is his kick. Keep an eye on his feet when he is swimming. He could be getting a great pull with his arms, but the kick is what drives the stroke, not the other way around. Small, fast kicks are much better than bigger, slower kicks that may produce more splashing. A good kick should be steady and consistent, and not sporadic.

(editor's note: this technique advice is useful for swimmers of all ages, and is one of the biggest technique issues in freestylers at all levels)