Hi there,
Unfortunately, yours is a scenario I hear about in abundance.
Marijuana is a bad idea for people with ADHD. It helps them to deal with the often-attendant anxiety (so they think it's great!) but it also further diminishes their working memory (but they don't care because they have less anxiety!).
Moreover, they can develop addictions. Never a good idea for brain function.
As far as convincing him to seek proper treatment, that is a complex issue. It starts with taking care of yourself and drawing boundaries, refusing to cover or compensate for him. Of course, this can often create more anger and alienation in the partner with untreated ADHD, and you have to be ready to deal with that.
There are more subtle ways of "getting through" a partner's denial about the effect untreated ADHD is having on his/her life and your relationship. You need to do just as you're doing -- finding your voice on this issue and seeking validation. Perhaps a support group would help you to firm your resolve.
Also, it's important to understand the nature of "denial" among some people with ADHD. It can be just not psychological but physiological -- that is, springing from the ADHD symptoms themselves. The more you understand this, the better you'll be able to find ways of reaching through.
If you can't find a support group near you, I encourage you to read the blog that I write for the non-profit devoted to ADHD: CHADD.
http://www.YouMeADD.orgGood luck!
Gina Pera, author
Is It You, Me, or Adult A.D.D.?