I'm assuming he has no problem getting hard - at least he didn't mention that as an issue, or part of the issue. As I understand it, there is no sensation of ejaculation - no spasms of any kind.
As we reach ejaculation, the cock will get harder than at any point during sexual activity. The balls will elevate. At the point of cumming, the cock will be as stiff as a 2x4, the Cremaster muscles clamp down on the balls, and the smooth muscles surrounding the prostate and those within the abdomen and perineum will rhythmically spasm, sending the semen on its way.
If none of this is happening and, instead, his cock goes limp as if he's already ejaculated, I can see why he's confused. None of the sensations of cumming and an immediate loss of the boner can lead a guy to wonder.
In addition to all the terrific suggestions made yesterday, I'll continue to research this. As Jean pointed out, he's surely not the only guy to experience this.