Believe him ???
You should THANK him.
You have another chance; work hard at this one - it will keep your record clean (and keep you out of jail).
I don't know what your charge was, but simple possession is a mandatory 90 days in county jail, and 'under the influence' is a mandatory 30 days in county. That's the MINIMUM, for misdemeanors.
On felonies, you're looking at the possibility of state prison for several years.
Why don't you shock your PD and tell him, sincerely, thanks.
It appears he did a great job for you (I, personally, don't beleive you should keep getting breaks, which you, obviously, aren't appreciative of, & you can't seem to stay out of trouble ... but, you have another chance - I hope you succeed.)
Enjoy the frequent Prop 36 appearances, the random testing, the group hugs and barney songs. The alternative is the jail time.
Please succeed - Prop 36 is an expensive program for the taxpayer.