My boyfriend comes quickly within a few minutes

My boyfriend comes quickly within a few minutes of being inside me, or goes soft whilst still inside me when he tries to hold an erection for any longer. We're 21, spend loads of time together, are comfortable around each other, have both had previous partners and have been intimate for 3 months now. He lasts for a fairly long time during oral sex, and we have tried many different positions (though some don’t work too well because he is a little small) but none particularly help. I have asked him how we can fix the situation, however whilst it seems to bother him, I don’t he views it as unusual. The only thing he suggests is getting drunk because it helps him last a little longer.
How can we fix things (without getting drunk)?

Caroline