At 12 weeks old I wouldn't worry too much about stay. I like to do a lot of stuff around the house when I bring home a new puppy, prior to starting puppy class, but they have such short attention spans at that age that expecting them to stay for more than a couple of seconds at a time is not really reasonable!
Right now I'd be focusing on teaching name recognition (an immediate whiplash head turn towards you when you say his name would be ideal), rewarding eye contact to reinforce focus, short recalls (on or off leash around the house - run backwards a few steps, encouraging him to follow, then whoop it up and give him a treat when he gets to you), and some impulse control stuff are all good. I like this game, and did it a lot when Halo was a puppy:
Using a treat lure to have him follow at your side as you walk around the house is a good foundation for heel or loose leash walking. I don't worry too much about using commands at this point, I mostly mark and reward behaviors I like. Keep training sessions VERY short, and fun.
I also do NILIF: Nothing in Life is Free
, which is totally adaptable to the age and ability of the dog. I teach my puppies to remain in a sit while I put the food bowl on the floor, and then I immediately release to eat. If puppy breaks the sit before the bowl touches the floor, I stand back up again. I usually only have to do this a few times before they figure it out. I make it very easy at first and then increase difficulty later. Here's Halo demonstrating at 15 weeks old: