I don't feel comfortable sharing too much my experience of a close family member with a drug addiction but wanted to come on and add my voice to those saying that you shouldn't feel bad to ignore and not engage with a great deal of posters asking you silly, unhelpful questions that you have mostly already answered. They clearly have no idea of the reality of living with somebody who becomes addicted to drugs.
My family member began young (14, county lines etc) and in their opinion it has ruined their life. In 30s now and still severely affected by it. It's devastating to love somebody in that situation.
The only advice I can offer is that drugs often are filling some kind of void. Kids who are more vulnerable to being exploited via county lines are ones who don't have the love and support they need to "make it" in the education system, etc. Of course this may not be the case with you, even kids with a lot of love and support can get sucked into drug abuse. Although you can try to influence them if he trusts you and values your opinion, you don't have power over his decisions.
You can only control your response, and you are clearly doing the best thing you feel you can under the circumstances. If sounds ok to me to say, "I love you and really want you to come home, but only when you have no drugs on you because I have to look after your siblings. They love you too and it could hurt them to see you like that", etc.
Perhaps you can try to identify whatever void he is likely filling with the drugs (love, boredom, low self esteem, academic anxiety, whatever it is) and work to adapt your parenting to try and counter that. If he won't go to counselling, you could go yourself. You could talk through your parenting of this situation with a professional, and work on being the absolute best parent you can be to your son. I know you are already doing this but it's so hard, getting some support for yourself and being open to anything you could potentially do differently could help.
Finally, in our situation, my mother & father going through a divorce (mainly amicable) and not working as a strong team with the right messaging definitely influenced how bad things got and how it has ended up shaping and limiting their entire life irrevocably.
Good luck xx