Off and on, I have run with both my daughter (now 13) and son (now 11). I really just try to read their interest level and mix it up as far as course and type of workout (nothing fancy, just mix in very mild intervals here and there). Then, of course, praise them after each run. The funniest part is that one of their favorite things about running is drinking Gatorade after the run... hey, whatever motivates them. This is probably my fault since I explained the science behind Gatorade, and so now they really think they need to replenish electrolytes and stock back up on carbohydrates, even after an easy 15-minute run. My daughter did her first 5k with run/walk intervals earlier this year (in memory of a teacher in her middle school), but she's only run a little since then.
In the end, I can't say they have caught the bug yet (hence the on/off nature of their running), but they definitely have a positive attitude about it, so I still have hope for the future.
Sounds like your son might be a little more motivated than my kiddos. Also, it sounds like you are doing it correctly... don't push, make it fun, and after he gets to a certain point, he will likely start to take it more seriously all on his own (especially after he completes a 5k). Kids seems to really enjoy race environments.