i think you need to approach this gently......not lock him in the basement for a week! the more parents PUSH on teenagers, the more rebellion is likely to arise. let him know your views on pot and WHY......and don't argue about whose it was etc. just that even tho pot may seem harmless, and maybe a lot of his buddies smoke, 14 is just too young to start experimenting with drugs.
this is a good time to share any personal experience you might have had with pot.
be honest
be firm
be fair