I would explore gifts that can give him an activity - something like Go ape, if he can cope with heights etc, or some other fun 'experience'.
"I should add that he is smoking and vaping with others who are supplying this stuff"
Is there anyone who can help to intervene here, school, a youth club worker or anyone.
What will happen when 'friends' stop supplying? He needs to think about that himself. Will he be encouraged to 'earn' money by dealing, stealing or whatever else?
LouisaRossini "I did make an appointment with the GP and he refused to attend because the time 'wasn't convenient for him' and he said it was 'a waste of time'. School have offered a counselling service but he's just not interested in engaging."
When my dd was being particularly difficult one night (not drug related) I did say that if this continued I would need to call the police. She was being violent with herself, throwing herself a round a bit, I feared she would her herself. I must say for me that worked quite well and she has not had an incident like that since.
If anyone thinks that was 'theatrical' on my part it was actually desperation! She went in the garden shouting at about 8.00 at night and I feared neighbors would call the police.
In your shoes I wold take advice from a kids and drugs charity about whether to threaten to involved the police or whether to involve the police anyway.
A threat to involve the police could be leverage to get him to do the course at school and go to GP. But do take advice.
Re 'Of course I don't like you, teenagers are not meant to like their parents!' Thats fine but you are the parent and you are in charge.
"It's really hard work to try and help someone who clearly thinks there is not a problem - he keeps saying I am being 'over protective!' He's very unpleasant"
It sounds incredibly hard, are you getting support from somewhere for you?
I really do think he needs to wake up and think how he is going to support his drugs habit when friends stop supplying, as they will once he is hooked. Does he not wonder why they are so generous? He needs a wake up call. It will be hard for you to supply it but a charity may be able to help.