I'm with Mr Tall on this one, and I do speak as someone who used to smoke a fair bit, but now only enjoy the occasional spliff. Skunk is an entirely different kettle of fish to the more usual grass/weed or solid we get over here, it's potency is far higher, effects stronger and more lasting. If you're ex-boyfriend is using skunk regularly, there are a lot of issues that could arise. It's potent and the potential side affects are far greater than if he was smoking "normal" grass or resin.
I think you did the right thing, for you. If you're not happy with him as a smoker, yet he seems unwilling to stop if you ask him, then you're best to walk away. Hopefully he'll re-assess his priorities someday soon and maybe think about getting help to stop. You could point him in the right direction but really, just like any other addict, be it drink, drugs or gambling etc, he has to first recognise that his habit is actually a problem, and it doesn't sound like he's at that stage yet. Good luck.