Depends. If I definitely 100% know it's explosives, I'd check to see if he has a deadman's switch in his hand. (ie, it goes boom if he releases his grip.) If I definitely 100% without-a-single-doubt know it's explosives and he has no visable deadman's switch or any other funny business, get as close as possible and fire at least three rounds into his brain stem.
I'd try to ID what the triggering mechanism is. If it's a simple push button type, I watch his hands to make sure he doesn't have his thumb over it. Muscle spasms and Mr Murphy will give odds that he'll clamp down on it under those circumstances. Or that his body will land on it. Or whatever.
If there's no visible triggering mechanism and his hands are moving freely, it's more than likely on a timer. In which case, after shooting him in the stem, I'd loudly encourage everyone to move in a very rapid manner and take my own advice.
If I have even an ounce of doubt, I'd simply notify security/911 and evac. Let the professionals handle it.