Your other two sons, especially your oldest boy, are readily aware that their brother is a talented athlete and receives special attention for his athletic success. Withholding enthusiasm or encouragement for your son's athletic pursuits neither serves him nor his siblings. It's every parent's responsibility to appreciate the natural talents of his kids. Make sure that you follow through on that responsibility for all three of your sons.
Allow your nine-year-old to view himself as much more than just a talented athlete. Notice all his special qualities, who he is (e.g., kind, compassionate, funny), not just what he does well. Don't forget to give your other boys the same attention. Celebrate all their respective talents and interests. They will come to know you as parents who really pay attention to the special qualities that they possess, while not valuing one son's gifts or skills above the others.
At this stage, it might be quite hard for your oldest son to feel inferior to his younger brother as an athlete. Some sensitivity to this potential problem is probably in order. I am sure that you will make all of your sons feel like you love and appreciate each one "the best." Thanks for writing.