More than likely this is not due to lack of muscle mass. Your son probably has significant stroke problems you can’t see from above the water surface. It could be that his hands are turned in a direction that doesn’t catch water. In other words his hands are slipping through the water instead of grabbing it. His hands may be entering the water in front of his face instead of more stretched out – causing a very short stroke. His hands could also be exiting the water too quickly, again creating a short stroke.

To see an example of world class freestyle stroke underwater I suggest you search Youtube for videos of Grant Hackett or Ian Thorpe (Grant Hackett video, Ian Thorpe video). Several have underwater video that show how their hands are positioned and how they “catch” the water. Watch your son underwater as he swims and look for major flaws such as the ones I mentioned above.

Your son’s speed should improve dramatically as he learns to relax a bit and lengthens his stroke. Each stroke will grab and push more water which in turn propels his body forward. He will swim faster with much less effort.