There's a lot of tension built up during sex--tension of the positive kind. Shaking can be the body's way of dispelling this tension, and giving into the awesome feelings that are taking over.

Sexual response is individual to each person, but trembling usually means that a person is getting pretty close to having an orgasm. It happens in women, too, and is sometimes referred to jokingly as the "Shaky Leg Syndrome."

So don't worry. The shaking just means that your boyfriend is experiencing an enormous amount of pleasure, which, in turn, means: Good job!

Take care.