My kids at 10 did play tennis, but local tennis options emphasized strokes, with minimal emphasis on fitness.
They both enjoyed soccer and basketball though, playing on school teams starting in the fourth grade.
Every summer they went to two weeks of basketball camp where they were run ragged playing full court games.
I am sure they got much more conditioning, learned how to play always in a balanced state, and were not afraid of playing to the point of being short of breath from playing basketball or soccer than from playing tennis.
Likewise, I'll bet your son gets a lot more conditioning playing soccer than playing tennis.
And soccer forces him to stay in the "athletic posture" of always keeping his body in balance for changes in direction.
Just keep a look out, and give him guidance if it looks like he is too often "standing tall", rather than in a balanced position when the ball is not near him. And keeping his feet in constant motion with many small adjusting steps, especially on defense. And knowing when to break out of a run to a sprint when the situation demands it.
You won't need the AP belt to teach your kid the athletic posture and won't need other conditioning if he embraces soccer played full out.
Athletic Foundation Intensity http://www.youtube.com/watch?v=ERo21...feature=relmfu
[I really think the main reason American tennis is in decline is that American kids don't play soccer like kids elsewhere in the world. And by the time kids would be old enough to have decent basketball skills to benefit from the conditioning, they quit the sport to concentrate full time on tennis.]