My son has had a sore neck for a couple days now .Plus he says his side hurts when he breaths.He hasn't been sick and is very healthy.What could it be?

If he had a stint of physical activity that was above what he usually has, he could just be sore from exercise. It could also be from poor posture, though this is not likely if this is very recent, and I don't imagine that painful breathing would be involved.