When can a puppy be outside alone/independent toileting?

My 12 week old cocker is great about going to the toilet outside mostly. He has never had an accident in his crate or play pen. The times he has had an accident it’s been because we missed a signal/took too long opening the door. He loves being in our little garden/courtyard if we are out there with him, but will whine at the door to come in if we are inside (we’ve once or twice kept him outside to clean up his accidents by the door so he doesn’t track them around the house). I was just wondering at what point we can pop him outside to toilet by himself without being present? Is that something that can be trained or do we have to go out with him for every toilet break until he is grown up? Thanks 🙂