I'm a specialist in this area. There are a bunch of botanical agents to enhance libido, and by virtue of their mechanism of action, they should increase semen volume as well. As with most herbs the daily dosage is usually 2000-4000mg per day for the "whole herb" and 500-1000mg per day of extract. I recommend combining two or more for best effect.
Wild Oats (Avena sativa)(arial part)
Epimedium (Horny goat weed)
Red Panax ginseng
Deer Velvet Antler (Lu Rong)
Cordyceps
Herba Cistanches (chinese herb)
Sarsaparilla root
Catuaba
Muira puama