Partners low sex drive ??

My partner smokes a lot of weed ... It doesn't seem to effect his personality but we hardly have sex now we've been together for 2 1/2 years And we do get along great but the sex has stopped and it's hitting me personally I feel like I'm not good enough or I can't turn my partner on and we have spoken about it and he says he just can't turn the tap on when ever I want it and I suppose I do get that, is there anything I can say or do to help my feelings and his as now he feels to much pressure to perform , but if I ignor and just wait till he's ready I end up waiting months :(