Remember that runners like Frank Shorter and Lasse Virun were 5 feet 10 and about 133 pounds. It is a great advantage when you have that kind of height to weight ratio. My son is 6' tall and 170lb. He was going to swim in the off season. He had started participating in a few triathlons last summer and did some biking in addition to his running.
Bodies can change alot in a short time in high school.
Look at the body types in your family, he may have put on muscle rapidly because thats what comes natural for him. I discovered that my running performance began to suffer when I was under 10 lbs. As for biking and swimming, I do not believe they harm running, but used intelligently can strenghten muscles which should be strenghtened to balance those used in running. The quads are a good example. The caveat here is overtraining, which should be considered as a possibility for explaining the decrease in performance.