It may be that the driver swerved out of control at the sight of a bicyclist careening in front of his car. Even braking suddenly can cause a car to veer like that, even at 30 mph. (Happens to me all the time when squirrels run out in front of me in our neighborhood. I know it's more dangerous to self and others to try to avoid small animals, but reflex kicks in, and there I am, braking and swerving. Luckily, it's a low-traffic area and I haven't caused any wrecks.) Did the witness say the driver was speeding?
If the bike's brakes failed, this is probably a no-fault accident. I'm not a lawyer, though, and you need one. Even if you finally get a helpful answer to your question, you'll still want to see a lawyer.
Was your son badly hurt? I hope not. You must be frantic. I can't imagine going through what you must be feeling. Let us know what happens.
Best wishes,
Arlee