Training

So. As I mentioned before, Asher's now working on improving his cardio and strength toward his black belt testing. One of our first steps has been swimming laps. Twice, now, I've taken him to our gym pool and just had him swim laps for 30-45 minutes. We're building our way up and intend to start actual "workouts" next week-ish. With that in mind, does anyone have any experience in this? Any recommendations? (I'll also consult with Noah to see what he did during swim team since I didn't watch most of those practices as they were at the same time as karate, lol.)
For what it's worth, Asher's already very active. He has PE on Mondays (at Options), gymnastics and karate on Tuesdays, swimming now on Wednesdays, karate on Thursdays, karate on Fridays, and sometimes karate on Saturdays. Plus, he's just almost always moving anyhow, lol. (Seriously, the kid doesn't even sit to eat oftentimes.)
I'm actually not concerned much with his physical fitness. But, the black belt testing has presented an opportunity for a relatively short term goal/push that Asher *wants* to take advantage of, so we're trying to work with him and fit this into the schedule. (I'd be lying if I didn't say I was a bit concerned that Asher feels this might help jump-start puberty. We've talked, though, and I'm hoping he understands that all the push-ups in the world aren't going to add much bulk to him at this point. Right?)
Anyhow. Beyond the swimming (and possibly mixing that up with running some of the time), we're going to introduce strength training on the weekends. And I'm working on convincing this most picky of eaters of mine that a change in diet will help also/more. ;)
In any case, we've started! So far, so good! (especially after we added the swim cap after that first time when the goggles got rather tangled in his now longer hair, lol)
Also, if you happen to know of any articles/books/friends that address exercise/training for pre-pubescent boys, please share! I can find plenty of recommendations for teens and men and plenty of advice to just let kids play...but could use advice on what to do with a 13 year old boy who'd like to workout but hasn't hit puberty yet and is on the incredibly small side. ;)