Why does he cum that fast?

So my boyfriend and I were heavily kissing and touching for a little while, so I went to give him a handjob. But I when I put my hand on it and did about a stroke he already came. This happens most times I go to do this. Why does this happen? How can I get him to last a little longer? We haven't had sex yet in the 9 months of us dating, so could that be a factor?