Preschooler scratching butt and playing with penis?

how the heck do i get my 2 year old to stop this!! it is absolutely disgusting!!! not the fact that he is playing with himself but the germs he is accumulating by doing this! he has no rash on his butt, just enjoys scratching it. i need him to stop now! i have a newborn and he goes from his privates to my daughters pacis and its gross. i obviously wash his hands often and if he touches her pacis i wash them but i just want him to stop!