Puppy Insanity

Ok so we found the cutest little puppy and tried to get him back home for weeks. We put up signs, went to the Pound and no one claimed him . Well after 3 weeks we have all fallen in love with him. Took him to Vet's, heartworm meds, shots, tags. Then the next week we had him fixed( boy he didn't like staying over night there lol) . So now we have a puppy.
My 4 yo has named him Lucky she wanted to call him Kentucky lol , she is so happy. BUT I am not so happy. All of a sudden he has began to use the house as his personal bathroom. Not funny. Why now ??? Been weeks here and he has been fine. I think he is letting me know that when I leave him at home he wants to go so he poos in the house to make sure I know this. hehe He has to learn real quick that this type of behavior will not fly.