How to lower my man's libido?

Well you're not allowing many options so perhaps you could let him take some of a woman's birth control pills and maybe those will dampen his libido some but he might begin developing boobs as a side effect. If he took steroids for a few weeks it would stop his body from producing testosterone for about 6- 7 weeks but that is only temporary... unless he repeated the cycle. But he could gain a lot of muscle and strength while doing such a cycle. :)