This is not an answer to your question, but perhaps some insight.
I'm a 20 year old male (a lot younger than all the other subjects) and am heavily addicted to Marijuana. I have a partner and suffer from severe erectile dysfunction. An erection will last for 3 - 5 minutes and then it will return to a completely placid state, and there's no getting it back again.
Erectile dysfunction for any male, regardless of age, is a massive burden for us. Not only do we carry the guilt of not being able to please our partner but we can't offload all the testosterone built up in the brain. Quite often I would storm out of the bedroom in anger or have an outburst with her in the bed. By outburst I mean punching walls and a lot of carry on. None of its nice so approach your partner with caution if you wish to discuss it with him.
The reason as to why we can't keep our erection is because when you smoke heavy dope, 20 - 30 cones a day, it becomes almost impossible to keep concentrated. Which you shouldn't have to in sex but Marijuana causes the brain to over think, hence anxiety and depression being a symptom of heavy dope usage. Once the brain is locked into thinking about something else, the erection goes and its too late to get it back.
Please note: this is NOT the woman's fault! Many women see the man loosing his erection as a fault on their behalf, but its not the case. It all in the man's head.
Today I quit for my partner and if there isn't an improvement in 7 days, i will try viagra.
A tip to keeping your man hard; give him oral sex or use your hand. When the man can see his penis, it's far easier to keep the erection. Why? Because during sex when the penis is in the woman and out of sights way, the mind is reliant only on feel and thus the brain is more likely to wander. Engage ALL the senses you can, sight being a big one.
If you man looses his hard-on during sex, get him to go down on your (if he enjoys it). Engaging the taste receptors should make him hard again.
Hope this was some help. I'll wrap it up with - erectile dysfunction is fu***d. I prefer to make love to my partner than be stoned. If he loves you and wants to express it, he should quit the green stuff.