Sorry to hear about your son's accident.
FWIW, an often overlooked issue is speed when puulling trailers. Bobgear, for instance, has a Safety Warning that the speed of the bike/trailer combination should not exceed 25mph. This is fairly easy to do on downhills. I had the same concern with a Mayacycle trailer - a speed induced wobble as speed approached 25mph once, as it approached 30 another time.
Of course, wobbles from poor load balancing can occur with panniers also.
Also, I've found that when braking with any trailer, and I've towed both single and double wheel trailers behind a diamond frame bike, applying the rear brake first and "pulsing it" tends to feel much more "positive" control-wise than applying both brake simultaneously or front brake first (which many people recommend).
Something to consider/be aware of...
IMHO, unless your son weighs less than my GF and what she pulls in her Bob Yak weigh (she -> ~120lbs, her cargo rarely over 35 lbs/plus trailer weight (14-15lbs?)), I'm not sure rider weight is much of a factor in the wobble problem. Otoh, she prefers using front panniers with 5-7 lbs of weight in each when towing a trailer. She says it makes her steering "more posiitive".
Sorry to reply to this so late:
Originally Posted by
dengidog
I know you had a problem w/yours, but did you speak w/the company to report it? Since nothing in life is 100%, maybe you got a duff skewer. It'd be a shame not to be able to enjoy your trailer for something that's "fixable" (plus saving others from the same problem). [/I].
I was one of the Mayacycle trailer testers, so, um, yeah been there, did that.