name changed as this is 

......but really need some good advice
I have a 17 year old son who started 6th form this year. Until half term, everything was going great; getting good grades in assignments/homework, volunteering for open evenings and it seemed like he was loving it then he got in with some new friends.....
since then he misses lessons, tried to drop a subject, got caught shop lifting, has stole from me on more than one occasion, started smoking (despite always being very against it)
he has always been very good at saying what the listener wants to hear (if the seed is planted) and as such his grandparents are convinced he is depressed (after they suggested it to him!, but I have arranged an appointment with CAHMS for next month through the GP), tells me one story, DH another (for some reason he seems to think we don't talk to each other!) and is general is throwing his life away
we had seemed to come to a compromise and things at home weren't so bad this last 2 weeks. Then today, he went to his 'friend's' house and wanted to stay over. I told him no, I whilst he was living under my roof, he was never stopping there as when ever he has gone there, then everything 'kicks off' at home (either arguments, stealing, lying or something). He seemed to take this ok......then a £10 note went missing from the kitchen and the only person who could have taken it is him - he denys it (of course!) and I did search his room
and I can not find anything. I have just noticed that another £5 note has gone missing from a change jar we have, but again, can not prove anything.
My gut is telling me drugs and I will be honest - I am staying awake so I can get hold of his phone when he falls asleep.....what do I do/how do I handle this???