I know the hell you have been put through I have a son who was on cocaine and more for years,alchohol as well.
It's sheer hell being a parent and not being able to help.
Firstly I do hope you will completely dis-regard the comments of member Bmp lksh, I've rarely heard of such heartless remarks being made regarding anything and least of all to a mother regarding her child. Obviously she/he has'nt any children ( if she/he has may god help them).
Unfortunately,drugs are so addictive and the urge to use them so strong like your son many fail time and time again trying to get off them.
I know it's hell for you, but please try and stay strong for him, he really needs your support. Yes, by all means try and find out if he is using something again,as has already been said the signs will be there if you look for them.
He will lie through his teeth you know that, because he is ashamed he has been weak again and let you down.
I truly wish you all the best and hope if he is taking drugs again that he will be free of this terrible addition in the end.
These drug dealers need to be hung,they are killing our children for money.The law should be changed.