I tossed BP for nine years and the philosophy of the club(s) that I was with had this purpose for BP:
-BP was not designed to pitch game speed.
-BP was designed to observe and correct poor batting postures and other issues by a batting coach. Thus, game speed pitches involved a batter moving too fast trying to hit the ball. Thus, if the incoming ball were tossed, but with a similar arm motion of a pitcher, the same incoming flight of the ball would produce the same effects as a pitch.
-Instead of game speed pitches, incoming BP balls were tossed more rapidly... one after the other. The batter would have to setup, get in the anticipation mode, make eye contact with the ball, swing ... then get ready to start the entire process all over again.
You'd be surprised at how all this pans out with a batter in the box, ball after ball. Subtle flaws in anything - stance, leg/hip/torso/shoulders/hands and swing signatures become really apparent, and quickly, once fatigue starts to settle in. Sloppiness, trying to over-do-it, trying to reach the bleachers instead of paying attention to the basics, floats to the surface ... and without fail.
So, my suggestion would be three fold.
First, give your body and throwing arm a breather by tossing BP, not pitching BP.
Second, get yourself some kind of protective screen to stand behind while tossing BP to your son. A shot right back at you ... third button down ... really smarts. Also, get a target to toss at, that's behind your son.
Third, stand no more that thirty feet from your son, thus allowing a demand of your son to recognize, commit, and swing with his intent to make quality contacts.
There are many videos on the web of how to toss BP. Most, if not all, will show you a "snap throw" as a method of tossing BP. This body motion by yourself - the BP pitcher, will allow repetitive tosses without straining. In fact, it can be good exercise for you.
Now here's a reality check - you're going to be sending baseball all over the place in the beginning, so get use to it and accept a steep learning curve for both you and your son. "Come on dad... get'em over," will be something that you'll hear a lot in the beginning. You'll also "ding" your son a few times ... well, more than just a few times .... ok, ok, ok... a lot of times. So, make sure your son has a batting helmet that fits.
It's a common misconception with younger players that a heavy bat will slug the ball out of the park. Not so. I'm not going to go into the coaching of batting, but for your son to take advantage of PB, have him use a light bat that he can swing with deliberate intent, something that is effortless to control. Remember, you're going to be sending one BP ball after another to him. Swinging like a backyard gate, just for the sake of swinging, is not what your son is in the box for.
Here's my final suggestion. Now is the time to get a video camera and video this time between you and your boy. You'll both get a kick out of this experience - trust me on this one. Later on when your boy is much older, perhaps on his own and your a grandpa, you'll both watch this video and your son will point out .. " here's that part with grandpa dinged me over and over...." GOOD LUCK GUYS AND ENJOY THE MOMENTS.