liz-most boys his age are notoriously weak in the upper body.If you want him to build up some real applicable strength without the use of freeweights(which I think would be premature at his age anyway), have him focus on either pull-ups or rope climbing.
Pull-ups are probably best to get him started with.Buy and install a quality pull-up bar and have him do many sets of 1 rep.You need to focus on quality here, not quantity.Make them full-range reps starting from a dead hang and pulling until the bar atleast touched his collar bone.This is a huge range of motion that most gym rats don't even bother to attain, but the greater range involves far more musculature than those silly half reps you see being performed by most trainees.
If he is seriously weak in the upper body, you may initially have to provide some assistance via holding his legs at the ankles and helping him through the sticking points.Eventually, he will adapt and be able to do them unassisted.You can also go to and purchase a set of mini-bands.These can be attached to a pull-up bar so the trainees feet are able to step into each band and they provide assistance at the bottom of the exercise.They are great for people who work out alone and they have many other applications.A set of minis will probably cost you $20 or so including the shipping.
Once your son has built up his pulling strength and can bang out atleast 10 in good form with a full-range of motion, look into purchasing a climbing rope.Rope climbing builds fantastic strength in the lats and arms and can also be great for conditioning.I purchased a 14 foot climbing rope from my local hardware store for $30 and installed it in my garage.During the warmer months, I substitute my pull-ups with rope climbing.It is a great exercise if you have the means to hook one up.
In addition to pull-ups, you will want to balance things out by having him implement either push-ups or dips.Medicine ball exercises are another option.