There are the ethical consumerism issues mentioned above, if that's important to you (personally, I wouldn't date anyone who didn't do their best to shop ethically, fair trade, renewable energy, ethical banking, etc. while being aware we can only do so much practically — but the intention is important), so check, it might be home-grown, after all.
Then there's the fact that it's illegal, if that's important to you (I'm not bothered by people breaking the law per se, as it's the ethical intention that counts for me, but I would still worry about the potential impact of being caught and on our relationship (e.g. if he had weed in your home).
Then there's the effect on him, which will impact your relationship: is he able to function in other areas of life, driving (certainly shouldn't be if he smokes weed regularly) , mood, behaviour.
In my experience, after work and at weekends is a lot and does have an impact on mood and behaviour, even if people who use that much claim those effects are positive.