The main thing you seem to be doing wrong is lack of supervision. At this age puppy should never be unsupervised even for an instant. When you can't watch him 110% of the time he should be confined, preferably in a properly sized crate.
Also, you need to take him to his pee pads if you're choosing to have him toilet in your house or take him outside on a schedule if that's what you choose to do. Don't expect him, especially at this young age, to tell you he needs to go or go to the pads himself. Just like a human baby he has no idea when he needs to go and does not yet have the muscle control to hold it when he does need to. Do you have everything on a strict schedule? Are you keeping a journal?