Your kid sounds amazingly similar to my son at that age. He started "training" (which really wasn't training) in the summer after 3rd grade when he was involved in a summer track program where I was the Director and distance coach. The kids met twice a week. One day for practice (they got a taste of ALL running events, so some printing, some hurdles, and a little bit of distance running, the furthest run of which was just a mile, and that was optional...no kid had to run further than 400 meters, and even there some would walk part of that). The other day for a meet. This was just a 6-week program. I never pushed him into running, and I did not require that he be a part of this track program. The fact is that he loved running and had since he learned he could.
The above said, I never went on any training runs with him at that age. He had some opportunities in elementary school when they were doing charity things or awareness things where kids got to see how many times they could run around a big field, and of course he destroyed everyone there and ran as long as they said he could. In 7th and 8th grade, he joined the cross country and track teams, and they ran on average 2 miles a day 5 days a week...that's it. He didn't start serious training (45 MPW I think it was) until the summer before his freshman year of high school. He later became a high school All-American and a D1 scholarship athlete.
Here's the important part -- I just supported his efforts. I did NOT coach him ever. I let his coaches coach him. I let his own interest and desire shape what he wanted to do. In addition to any natural talent an athlete has, the most important thing is their enjoyment of the sport. Yes, they have to train, but if they don't love it, they will cut corners. You need to do all you can to allow this to be HIS thing, not something he does to please Daddy. Kids get older and some rebel and want their own independence, and it's easy to decide to put something aside that Daddy wants the kid to do when it is time for them to carve their own path.
I am not for any serious training until after 8th grade is over. Success as a runner in middle school is mostly due to one of two things -- either the kid is physically mature beyond the others at that point, or he is just better trained. You can get a very false sense of where a kid is if they are beating everyone by a lot in middle school, because once high school happens, everyone catches up physically, and all the good ones are training as they should too.
My advice is that you should not do any structured training with him at all. Fine for him to be a part of the winter CC thing, but his "training" should stay within the parameters set by whoever the coach is for that. Don't do anything extra. Your kid will be into the summer before his freshman year of CC before you know it. He can start serious training then.