My son is 4 and he gets pain behind his knees at night. what could be causing this?

Night Leg Pains: Children commonly begin to experience "growing pains" in their legs around this age--around their knees or over their shins. There is no known specific cause for these, but they are common. They are different than the common aches and pains of normal active children. As long as the child has the pains only at night, and has normal function of the knee in daytime, s/he will outgrow it.