Sounds like he may be dependant on it(don't buy the hype, you can become dependant on pot[psychological addiction]). If it's something you won't budge on, and he is adamant about it, break up with him. It really isn't worth it sometimes. This is under the assumption y'all don't live together or have a kid. If he lives with you, make him move out. If you live with him, start packing and move back home.
*I am passionately against drugs of any form.