Pothead Boyfriend

So basically my boyfriend and I have been dating for almost 3 years now. He's been smoking weed ever since we've been dating. At first I was completely okay with it considering I was only 17 and was just having fun like a normal teen. Now I'm almost 20 and he's 24 & he seems to think its okay to still smoke pot. It's not like it's a huge deal to me but recently he acts as if he needs to smoke in order to do anything. We can't even leave the house to do something simple unless he smokes beforehand. Whenever I bring it up and tell him that it's a problem, he says that I'm overreacting. Normally, I would smoke with him just for something to do but lately I'm just not in the mood and want to focus more on my future. I don't want to give him an ultimatum because I feel that it's not right. What do I do? Am I overreacting? If it comes down to it, i don't want to lose him over something like this, but then again it is something that is going to affect our future. Opinions?