My boyfriend finished far too quick 😞

Been sleeping together for 3 months and I thought that would be long enough for him to master making me finish or even lasting longer but he always finishes 3 mins into missionary and then our sex is over. We do a lot of foreplay beforehand so it’s not bad and I have told him that it’s not always enough for me, yet neither of us really know what to do. Does anyone have any advice?