The 19 year old has committed crimes, buying alcohol for a child is illegal. So is buying cigarettes for a child. There is a possibility of a sexual relationship with a child. So at least two crimes. Possible shoplifting too.
Tell 19 year old and their counsellor to have no contact with your son.
Complain about the police. and escalate it.
I'd ask for some legal advice about getting some form of non molestation / restraining order to stop the 19 year old from contacting your son, you and anyone the family.
A for your son, have you discussed his running away?
I don't mean just, 'don't do it' but gone through various scenarios with him? Some people just don't have the imagination to understand the danger unless it is spelled out.
EG saying never go with a stranger doesn't always compute when that stranger is a child or a woman or is walking a dog.
So you have to go down the route of what if it's snowing and someone offers you a lift?
So explain what the various dangers are to him and you when he runs away.
That running away doesn't stop the problems.
Make a plan together (him, parents, other children) for him to do something instead, does he have a grandparent near by? A trusted friend who's parents you know so he has somewhere to go when he feels he needs to get out, even a time out card which gives him 30 mins in his room without you asking any questions.
Just some way he can be safe.
Sorry I can't think of anything more, good luck with this.