Some of the people blaming Beachywave should think about the fact that this can happen to any of your children. The op's DS is a victim.
County lines is insidious, targeted and relentless. There is no victim profile that 'fits' with the one a lot of Mumsnetters believe. Children from 'nice' homes can and are, targeted in the same way as those whose background and home life may be less 'stable'.
It can start off in a small, seemingly innocuous way. A child will be approached in a 'friendly' manner, outside school, waiting for the bus, playing football in the park and maybe have a chat to establish a connection. They might then 'accidentally' bump into said child again and (having already established the interests of the victim; gaming, trainers, football, mobile phones) say 'hey mate I've got a brand new pair of Nike whatever is 'in'. I ordered them accidentally etc etc, you can have them, free, no charge'. If the victim accepts, the seemingly innocuous and kind 'gift' that's the turning point, they're now 'in'. The victim will them be approached at a later date and, still in a friendly, banter way be asked to do a favour. 'Hey mate, how are the trainers? Great, I'm glad you like them. Could you do me a favour? I just need you to take this (envelope/parcel/money) to x address. It'll only take you 10 minutes and you'd be doing me a massive favour'. Child victim feels they have no choice, they don't know at this point it's (drugs, money etc) and agree.
There are now two things that can be leveraged against the victim and 'debt bonding' begins. You did xyz for me, you can get in a lot of trouble for that, now you need to do xyz again otherwise we'll do xyz (hurt family, tell police (they won't), beat you up etc etc etc). It can spiral quickly and the victim is soon in deeper than they can comprehend. They can't tell anyone (parent, sibling, teacher, police) because of the fear of possible repercussions. They are a victim though and should be treated as such.
The courts will look at the level of 'role' your son played (lesser) alongside the class of drug. Him playing a lesser role, his age and being a victim will be in his favour in terms of sentencing. Your family are being unkind and that must be very hard to bear alongside you supporting your DS as well as keeping life going for your family. In your situation I would write them a letter, explaining how your son is a victim, how you both need support now more than ever.
Hopefully your DS will get a community order. You should not blame yourself or your DS. He most probably will be afraid of all sorts of repuccussions to him/his family if he says anything more than admitting guilt. He will have been 'briefed' on what to do if he gets caught and what will happen if he 'grasses'.
To all of the posters who judge the OP and think it won't happen to their children, it can happen to any child. I suggest you read up on County Lines, signs to look out for and what to do if you suspect your child has become involved. They often do use children from 'nice' families because they'll have more to leverage against.
County Lines is real and scary and you don't have to live in an inner city for it to affect you or your children. Educate your dc on the approaches people take to recruit them. Think about when your DC were little and you knew something was worrying them. They finally tell you that child A at school said if they didn't do x then a monster would break into their home at night and do xyz. County Lines is the real version of that and OP's DS was likely 14/15 when it started. He is a child who's a victim of an over arching criminal and ruthless gang. He and the OP are living the real life version of a monster doing xyz if the OP's DS does/doesnt do xyz.