The only method that truly works is catching the accidents before they happen or as they happen and correcting them at that moment.
Your puppy is still very young- you need to have some set-up like an expen to put him in when no one can watch him. When he is out, you need to have your eyes on him at all times- as soon as you see the signs that he wants to eliminate or as soon as he squats- clap your hands, say NO, and take him to the pad (or outside, whereever you are training him to go)- if he eliminates on the pad or even just finishes up on the pad you give him lots of praise and a treat. Also, be proactive- every 15 minutes take him to the pad and give him a chance to go, if he does, again lots of treats and praise. It will take a consistency and time but eventually you will see progress. If your BF is not on board with this plan- keep him (either the BF or Aiko, your choice
) in the ex pen while you are gone, so there are no chances for accidents.
Punishing a dog for going in the wrong place, only punishes them for going in their eyes- they will start to hide when they are going. Then you are left with the situation where they are peeing under and behind objects and you don't know until you start to smell it.