Ahh, bless!only 12 weeks old? I doubt he is spraying as such. Some puppies have weaker bladders than others. Hannah is 8months and a work in progress! Sometimes the time you take to open the door is just too long, especially if they have been busy playing and not realiesed they needed to go.
Just take him outside loads and treat and praise when he goes in the right place, he will catch on and his bladder control will improve..