Have you considered that your bf has been slipping into a depression and smoking more weed because he has more need for something to make him feel good? It's a classic case with most people who smoke weed, including myself.

But right now it may be good for him to quit for a while. Not because weed is making him depressed, but because he's using it as an emotional crutch with his depression, and when you constantly hide away in a bottle, a joint, a needle, junkfood, or even just hours of television, you're training your brain to just avoid what which makes you miserable instead of learning how to deal with it. I think the best thing is to find his solution for a better life through his own power.
How? nobody here can give a definitive answer but I could advise a few things. Learning meditation and getting more physically active would definately help, as he needs to learn how to push himself and still feel good about it.
He could also start taking large quantities of Omega 3, 6, and 9 fatty acids with sublingual vitamin-B12 tablets (this actually worked really well for me after taking them for a month).
Then, I'd advise helping him get a new perspective on life. Do something like go camping for a whole week, something that cuts him off from the basic comforts and indulgences of every day life, because these can be a real problem in numbing the mind into a comfortable yet miserable rut.
Then, and this is a real simple one, there's breathing. Believe it not, chaning the way a person breaths in every day life can significantly affect intelligence, mental clarity, quality of sleep, and depression. Quite simply, get him (heck, do this yourself) to mentally focus no breathing deeper and slower at all times. You'd be surprised what a difference it can make.

Best of luck to you.