You can't force him as he'll dig his heels in, snap at you and tell you he'll do it in his own time. He knows full well he needs to make that appointment, but he hasn't reconciled quitting in his head quite yet.
Ex 20 a day smoker, now quit for 1 year 2 1/2 months.
I've been encouraging my DH to reduce gradually over the last 4 years. Went from about 10 a day to 2 a day over that time period. Then I got pregnant and with zero nagging he has reduced to 1-2 per week max.
My DP promised to stop smoking then couldn't and ended up lying about it which caused no end of problems. Let him do it in his own time, don't nag or pressure him. It may seem like things aren't stressful at the moment, but if he's anything like my DP he'll be trying to hide how scared he is. Just let him do it in his own time.
I feel for you Ellie, I'm moving in with my boyfriend when my uni contract expires, but until then i'm living in a house with five lads who love weed! It would be very hard for any of them to give up, it must be hard for your OH considering he's dealing with the stresses of becoming a dad etc!
Smoking weed doesn't make him a terrible person madlizzy, and I know what the cultures like amongst young guys, when all your friends are doing it, its hard to give up despite the health implications etc! And i'm sure he's not thinking about it in the sense of being selfish, to be fair in terms of cost etc, it is much cheaper to sit there and smoke than go out and get drunk! But in terms of his mental function I suppose he is. Mental health impacts don't affect everyone though!
The best way is to not allow it in your home, then if he does it at his friends houses hopefully he'll cut down and step up a bit when he's a dad!
When I found out I was pg at 8 weeks DP said (without me saying anything) that he would give up weed and cigarettes. It took about 3 months of struggling but he seemed to have managed it. But then he'd go out and have a couple. This has escalated and now DD is 8 months and he's smoking loads in secret he thinks. He's also started working with a friend who's encouraging him to smoke weed again (probably to annoy me, or telling him he shouldn't do what I want). It's really annoying me now!