Can you let him sweat it out in Juvie for a little while (if only a couple of hours)? Rather than ransack the room, see if the cops will send a sniffer dog over and give the place a once-over. Less intrusive, and better all-around. Then you can ask him to level with you: do you have anything else in the house? See what he says.

Be glad it's just weed. It was never my thing (couldn't afford it even if it was), but I can think of lots of worse addictions! Good luck!

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1992 300D 2.5T
1980 Euro 300D (sadly, sold)
1998 Jetta TDI, 132K "Rudy"
1974 Triumph TR6
1999 Saab 9-5 wagon (wife's)