As far as smoking MJ just tell him to be honest and say he smokes , But tell them he only smokes a joint about every 2 or 3 weeks . At this point all they can do is talk.
And honestly thats what they want to do intergate him to see the extent of his 'criminal actividys' to see how close they want to monitor him . No sense lyeing your ass off it will just raise flags , just dont admit to anything else and have backround stories made up - My hobies, the direction I am trying to move my life into, School, work experiance, whatever.
if your honest about smokeing now , the more likely they are about letting you slide on a dirty test later. You just dont want to tell them you smoke reguraly , if you do that their likely to send you to intensive out patent 'rehab'. You/or how ever is most likely/ HIGHLY likely to be sent to out patent anyway since it's a drug charge
My best advice is just to be very frendly and polite . only admit to crimes you have ben charged with / otherwise your no angel but a good polite ,cleancut man
Also if you are truely a criminal (no judgement hear) read up on body langue and how to use it, and how the LEO's use it -Its a masivly usefull skill in all aspects of life anyway