How do i get my dog to stop pooping in the house?

I have a 4 month old rottie sheppard mix who i can not get to stop pooing in the house. i've had him since six weeks old and started training him then. He never pooped in the house until recently he would only pee now it seems like the opposite. I know he is still to young to expect him to be compleatly house broken but how do i start to fix this problem?