The whitish substance you see is called Smegma, which results from secretion of oil glands of the skin as well as dead skin cells .It accumulates under the foreskin. The foreskin should be pulled back gently and washed thoroughly with mild soap and water during a daily bath.This will prevent it building up and possibly getting infected (in which case it,if left untreatd, will become foul smelling and can lead to more serious infections of the penis). Accumulation of smegma is less common or rare in the circumcised. If after attempts at proper hygiene have been made and the accumulation continues, let him be reviewed by a doctor(paediatrician).