They do come in different voltages, but I think the first thing you have to be sure of is your terms - "stun gun" vs. "TASER".
A stun-gun is the little hand-held unit that you actually have to make contact with on your attacker. A TASER is what the police use, and has a pair of wires attached to 2 small darts, which you "fire" at your attacker from up to about 15 feet away.
TASERS also require a felony background check before purchase, whereas stun guns do not.
The better choice is probably the stun gun for personal self-defense. They range in power from 100,000 to around 800,000 volts (very low amperage), but depending upon the quality of the gun a lower-voltage model may be more efficient than a high-voltage one.
I'm sorry for the trouble with your son - can you call the police? I'm assuming he's over the legal age of consent ...