The cannabis will not be helping his depression. My first husband smoked increasing amounts of cannabis until he wouldn't leave the house at all as it interfered with his smoking schedule. He became very depressed/paranoid/irrational/angry, and wouldn't admit it was the cannabis that was the cause. He said that if he couldn't smoke cannabis he would have to drink 8 cans of lager every night.
Unfortunately, there's no easy solution if he's not willing to change. I tried everything with my husband until I had a nervous breakdown myself and left him.
If you can get your husband to see that the cannabis is probably making things worse and that he should stop smoking it, the Prozac might help more.
I agree with notjustmom about the counselling - talking to an objective third party might give hima kick start - my husband always thought I was just nagging.