Feeling torn...

I have a 10yr old from a previous relationship and a 2yr old to my current partner of 6 years...we are happy and he is such a good partner and father! He would never cheat and treats my boy like his own...I just have one problem..he smokes weed. He never does it around the kids but when it comes to needing it, he just buys it whenever. That's when the arguments start, he thinks he earns the money so he should be able to spend it as he pleases. I am a stay at home mum to my 2yr old but I also work part time at night at a bowling alley so am trying. Am I wrong in feeling upset? Why does he think like that? I'm 28 and he's 30...