Low libido. Hard with a loss of libido?

My partner and I have only had sex 4 times in the last 8 months. we went from having a great sex life to an almost non existent one quite quickly. he has been a full time pot smoker for about 8 years which I feel he self medicates to treat his anxiety. he still currently gets erections but he says he has no desire. I always thought they fell hand in hand with each other. I mean, surely if he's getting hard there's still desire? considering hes only 24, I don't know whether to blame weed or whether it's a stress/anxiety thing. it's not something he willingly talks about. how should I discuss this with him and encourage him to see a doctor so at least we can rule out it being a medical problem? is it possible he can still get hard with a loss of libido? thank you