He does hold out for a while, at least, until the pleasure builds into something like a soreness. Even then, it takes him slowly, no real change in pace, a touch faster perhaps but still slow and dragging and deep, savoring every sensation. The peak rolls through him more like a wave coming to shore, the peak definitive but fluid, blood rushing in his ears. He sobs his way through it, not exactly crying but gathering Thanatos close with a noise more like a wail, and he suddenly has to move, almost violently, as though all the slowness has found sudden motion in the climax.

He grips Thanatos's thigh hard enough to bruise and rolls them, hard, even as he can feel his own muscles starting to want to go limp in stunned aftermath, so that the other man is sitting astride him, can collapse against his larger form instead of being crushed by his. It might also let him stay inside him longer, particularly if Thanatos hasn't found his own release yet-- he might be able to pursue it, while Gustavain can't control his limbs for a moment.

Above all else, it seems imperative that they stay together, pressed close, in whatever way possible.