Back in the day when I could afford suplements, I thought that extra zinc might not increase spunk volume but it certainly made it look like it was more, because the zinc makes the spunk thicker, whiter and therefore more noticable.
What you really need is a prostate vibrator. P massage, slowly building up for 2 hours, and you will shoot like a porn star.
Sooner or later someone will mention testosterone supplements. For a few men this is worthwhile, for most the risks outweigh the likely advantages. Only a doctor can tell if your T level is low, dont take this stuff unless you actualy need it.