Could it be that you are catching him in the act when in the house and he shuts is pee stream off and than resumes again an again each time. Pups at that age like to wonder around for the perfect spot to potty. So if this is all happening in a 15 min time span my guess would be that he just isn't finished.
Or a urinary track infection (UTI)..
You are welcome to view my techniques on potty training if you like. I've trained all my pups using the same thing and I've had great success.
www.dawnsyorkies.weebly.com/training-tips