Hi there. I'm not sure I can help but I went through a similar period myself.
I'm 23 now but when I was 16, I was smoking weed heavily, drinking and getting myself into a lot of trouble. I didn't have a relationship with my mum and my dad's parenting was inconsistent. I refused to sign on because I found it embarrassing and didn't want my friends who went off to uni to hear I was a failure. Eventually, my dad had enough and stopped funding my habit. I didn't have anywhere to turn and my mum suggested me cleaning for my grandad so I could earn some money. I did and it slowly started to make a difference. My grandad was set in his ways and I had a great deal of respect for him. He was always encouraging me to get a job and better myself and so I started looking. I signed on and I hated every second of it. At first I lied and said I was looking for work but eventually I realised I was lying to myself. At the point I realised how far I had let myself sink. I got a job and cut down how much I was smoking. I started exercising and releasing my anxiety and stress at the gym. There were other contributing factors that made me wake up and it was a long road but I got there. I wanted to change in the end so I made it happen. I've worked my way up to management now and have a 9 month old daughter. I don't recognise who I was before.
I didn't have the best childhood and raised myself from a young age. I had low self- esteem and bad anxiety. The majority of my grades were good but school was always a negative place for me for lots of reasons. I could never handle discipline. They are some of the reasons why I acted out.
I don't know your son so I couldn't say why he's behaving the way he is but maybe look at why he wanted to escape. Weed is recreational and fun but when it becomes a a daily use it's usually an escape or coping mechanism. Having a baby has put the pressure on him intensely and has probably made him feel more useless.
Maybe there's someone in your family he respects that can talk to him? Trying to get outside help could be good but for me I would have rebelled even harder.
Is there anyone who he could take up a hobby/sport with or even go to the gym? It'll make him feel better about himself hopefully.
Do you still give him money? Maybe change your attitude towards that, if you do and give the money directly to his girlfriend or buy things for the baby but don't give it to him. It seems harsh and he may act out to begin with but he has to learn that he's got to make his own way.
Maybe he's got some decent friends that have gone off to find work or go to uni that could encourage him?
Lastly, maybe try approaching him at a good time and talk about doing an apprenticeship. Be careful how you word things. So pressure just a conversation. The money isn't great but maybe he'd be interested in learning a proper trade instead of working in something he would deem 'lame'. Try and see what he's interested in and talk about what kind of money and prospects he could be looking at in a few years. I found that a good motivator. I knew I wasn't going to be on instant good money but knowing I would eventually made we work harder.
He needs to step up as a father but he can't do that until he's sorted himself out so encourage him to spend time with his son but try not to demonise him for not being the father he should be. Not saying that you are by the way. He needs a wake up call from different people and different factors in his life and hopefully he'll decide he needs to change.
I hope something I've wrote above helps. I felt so much pressure to be instantly successful at 18 but your son needs to know that he's got a support system and that once he starts making changes, he'll feel better about himself.
Stay strong.