My 13 yr old son has to fine is own summer workout

Join an AAU track team. Go to their website to find a local team. It will likely be coached by a local HS coach, but they usually are allowed to give instruction since its open to all schools in the district.

My son will be in 7th grade next Fall and is getting his workouts from the HS track coach. Currently, he has him doing anywhere from 8 to 12 400 meter repeats (after warmup) which consist of the following: Run 300 meters, slow jog 100 meters, repeat. Then has him do 200 run, 200 slow jog, repeat.

The HS coach is the one that approached my middle school son and a couple of his peers about joining his HS workouts after watching them compete in the middle school. Not sure if that breaks any rules in Tennessee.

Get him doing a plyometric workout. You can find plyo drills almost anywhere. PM me if you need help.

OA5II,
I am good with the track advice, but you need to be very careful that he doesn't overwork himself; his body is still growing and that uses a whole lot of energy by itself. Sounds like he has quite a full plate; at 138 lbs, he needs to eat as much as he can as well. Finally, on the plyos, I would recommend a lot of caution. At 13, the joints haven't fully developed and plyometrics can be very stressful on them.
OBC

Stay away from plyo's at his age.

All he needs is a medicine ball and his own bodyweight.

I agree with staying away from plyometrics. They are not "bad" per se, but people do not generally understand how to do them properly and even what a plyometric exercise really is. If you must do plyo's, read the book "Jumping into Plyometrics" by Donald Chu for some good background and baseball specific workouts. For a 13-14 year old, you can make a lot of progress with:

1) Running (sprints and endurance)
2) Body weight exercises such as planks, pushups, pullups
3) Light rotational and core strength medicine ball work

although not very common (as far as I can tell), kettlebells are a good way train core strength and cardio very efficiently. Keeping it fun and motivational is really more important for kids at this level in my opinion.

You do have to be very careful with plyos.

Would add this suggestion; real cheap to get the components and it can offer a great workout: find a old duffle bag or two (smaller would be better for what you will need it for)and some fairly durable garbage bags. Buy a bag of small pea gravel (sand is used a lot, but IMO, leaks through pretty much whatever you put it in) and fill the garbage bags to about half, tie them off well and throw them in the duffle. Weight the duffle to a weight that you think will give your boy a challenge (less is better at first, you might be surprised, the instability of the weight really calls a lot more muscles into play). You can search on the internet under "sandbag training" and pick up a bunch of exercises. Great for the entire body (which, IMO is what you should really be trying to develop) and it strengthens the hands like you wouldn't believe. If it were up to me, pretty much every off-season baseball program would include these kind of exercises...