The opposite of whatever you are doing at the moment. You get the guy so fired up he lights the rockets on July 2nd. Would you mind teaching classes to bored housewives?
About 10% of guys get this problem regularly, but it happens to every guy on Earth at least once in their life. And it is a completely soul destroying blow to the ego whenever it happens. The hose running when she ain't on fire is not the best thing ever, by a long shot.
It does get better as you get more practice in, even for the habitually trigger happy, and learn a bit of self control. Having a drink to take the edge off apparently works too, as a short term fix. If the worst comes to the worst, there is always medication, though I hate recommending that stuff. Doctors see enough humorous sh*t without that.
Practically speaking, what are his triggers? Full on intercourse, or you in heels and a smile? For most guys who suffer from PE, it is the full intercourse that triggers it. So, while he is learning restraint, get him to ring your bells with hands and tongue, before saddling up and ringing his own.
If it is just you in heels and a smile that causes it - I want a picture. Right now.