I feel guilty now mentioning that video.. but hes a grown man and should expect his mum to be worried. Its not like you have done something bad. Your doing what any mum would do.
Does he have a partner or close cousins? Could you not ask them if hes using? But then again will he go mad if your asking?
It does seem like he is on coke. But you dont really know till he admits it or you find it on him. And i wouldnt want to see you lose your relationship over your son on assumption.
Maybe.. this is just my opinion.. just say something like.. ok.. im just worried about you.. i wont bring it up again.. i dont want to lose you or my grandaughter, but im here for you no matter what, and you can tell me anything… thats just my opinion.
But i hope it goes well for you.