
Have you just been leaving the door open for him? I know some who do this when they get pups in the summer and all seems well until they start closing the door when the weather turns and they find out the pup hasn't learnt to hold it and let them know it needs the loo, so they have to start again.
Has it been raining where you are? My boy hates the rain and doesn't even like walking in set floor outside, incase it will melt his paws, so when it's wet out there as a pup he would be like "nope I'll just go in the kitchen ok?"