Since he still wants to swim, any suggestions to help him to keep motivated and focused until he grows?
Swimming Guide: Since you mention your son is still improving, I'm guessing that trying to focus on setting goals for individual is not too affective right now? It is very common for young standouts to get "caught and passed" by other swimmers as they each experience different growth spurts. There is an old saying that "you can't make the person next to you do worse, but you can make yourself do better."
Apples and oranges - you shuldn't compare your son's times with another person, have your son compare them with his own times. I know that you are not the ones comparing him to others - he has to learn to whom he can and cannot compare himself, and yes, he should aim to be the best, but he can't stop someone else that's a giant! That's not realistic in every case. It is possible to overcome someone of superior size; there is more to swimming than height and strength. Pushing to exceed your limits is to be encouraged, but it must have successful results every so often to keep its motivational powers intact. And remember swimming is a sport - a game - and keep fun in the mix!
The problem, as you are experiencing, is in growing, racing, and placing. Competitive swimming is competing, no doubt about that. But there are times (like now) when the pool isn't "fair" to everyone. That equality, from physical maturation, doesn't happen until boys are "physically men" - age 19 to 23 for most males. There are swimmers that just "have" something that makes them better, no matter what age - they have a gift, they have incredible talent, or everything is clicking all of the time. In the mean time, they are all in different stages of development.
Help your son learn to understand this development. He might not accept it; I hope he doesn't - that's OK - knowing but still trying is the goal; not realizing and giving up is not.
We want him to be excited about swimming and what he is capable of doing - and what his work now can lead to in the future. One path to take is more time and technique oriented, and less focused on placing in races. Use races as tests for development; growth within himself. Then, as he grows physically, he will always have the ability to push himself, even if there is no one else to push him in a workout or a race. Every practice and every race, your son can set different types of goals for himself. "Today, I will:
- hold a 1:10 average on this workout set."
- split the last half of this race faster than I ever have in my life."
- hold great swimming technique further than I did the last time."
- perform the same time but take fewer stroke cycles."
- work on my weakest stroke in the individual medley when I have a choice of strokes."