Lots of useless advice from people saying "ban" him from smoking weed. Sorry. How do you physically stop an addicted adult from doing something?
I sometimes see advice aimed at parents with teenagers on here and think posters must only have younger DCs or those biddable, academic girls.
Ultimately if the OP kicks him out, he WILL be street homeless unless the OP is prepared to fund rented accommodation. Even then, what landlord is going to rent to a jobless 18 year old? SS won't give a shit; he's an adult. He'll possibly end up in Salvation Army or YMCA hostels with ex convicts and hard core druggies/alkies. It could be a wake up call or a slippery slope?
I don't know if there are any solutions.
Speaking to your GP is a reasonable idea as is getting a referral to drug charities or MH services. What support is available in your area I do not know and as always services have been cut and your DS will not be a priority as he's under the radar.
Would he consider something like a Working holiday in Australia to get him out of his bedroom and environment. It sounds like he's in a horrible, depressed spiral of addiction and misery and needs a way to get out. Try appealing to his better nature regarding the drugs, Google "child modern slavery in UK grow houses" something like that and show him where his weed actually comes from.
Drugs are an absolute scourge on our society. I'm sorry OP. I hope he comes through it.