If he was on probation at the time, and a condition of probation was to submit to drug tests, then he had no choice, and if he did not submit to the drug test, he could've been violated for a refusal.
But I would simply add to what the first person said. In most states - mine certainly - a...
Get a lawyer. It is well worth the cost for a DUI. At the very least, the lawyer can negotiate a better plea than you could get on your own. I'm guessing CA, like my state (NC), is a zero tolerance state for people under 18 on provisional licenses.
You want to go up with a plea agreement...