I understand that in tantric sex, for a man, keeping the semen in is a requirement for keeping energy levels up. I can understand because I have... a friend... who mastrabates all the live long day and he complains of being exhausted especially during excessive bouts. Is this true? I mean about keeping energy levels up, and if so, how should one keep it in...something whose nature is to go out?
This is a deep question because there are lots of kinds of orgasm. One way to divide them is by whether they build energy or deplete energy.
When we don't move energy during sex we don't build energy. Typically, in energy-depleting sex the goal is to orgasm, a man achieves it by stimulating the penis and a woman achieves it by stimulating the clitoris, and when it's over we're sleepy. It can be wonderful in a million ways but after the orgasm we tend to feel drained.
When we intentionally build energy during sex we create new energy and afterward we feel energized. Sexual energy work isn't easy to explain in writing but here are a few pieces of the puzzle:
Energy can be aroused through direct and indirect stimulation of the P-spot in men and the G-spot in women. The P-spot and G-spot seem linked to energy creation in ways that the penis and clit are not. Because the P-spot and G-spot are both small balls about the size of a walnut, there are lots of methods your friend can use to stimulate his spot from different sides and angles.
Holding off on orgasm
Men and women can build energy by holding off on orgasm. Common ways to hold off are to breathe deeply and slowly, pull away and stop all movement, or change mental focus to something not sexy to cool down. There is no magic number for how long or how many times to hold off on orgasm to increase energy, but your friend can experiment holding off longer and longer until he knows what works best.
Kundalini and the chakras
We can create energy during sex by physically and energetically moving energy from the base of our spine, our root, up through our spinal column and through the head. There are variations on this theme, discussed under such online search terms as "tantra" or "tantric," "kundalini," "chakras" and "sexual energy." Most techniques will involve a combination of meditation and undulation of the spinal column, or snake, from the root through the crown and feeling the energy build and move.
It's difficult to find quality information about energizing orgasms. If your friend is in the Austin or San Antonio area, an excellent place to learn is the Toltec Center of Creative Intent when they offer Sexual Integrity events. If your friend is interested in tapping his life force through sexual energy creation, I hope that he explores, soaks in information from various sources to see how they differ, and has fun experimenting to find out what path serves him best.