you have to walk outside with puppies a lot. (very short walks tough) But he can't hold it up yet, so just go outside a lot. He will get used to do it outside.

If he grows up and is over the time that he should be potty trained, and he still isn't, be a little more harsh. When he pees on the floor push is nose in his own pee. This maybe sounds really harsh, but you are the boss. And he will not hate you for it. This works better than saying "bad dog" , because he maybe doesn;t understand why he is bad. Especially if he peed there 30 minutes ago. But this way it's pretty clear what you mean.

And if the dog goes to the door and sits there (sometimes they do very sneaky), go outside with him! Don't wait untill you finished reading that page, or for the commercials on tv. If he gets older he can be more patient of course.