As others have said, crate him.
Extra advice I can offer as well as what has already been said - if he barks, cries, howls while you are out of the room, leave for as long as you can. You don't want him getting too stressed, or neighbours becoming stressed either.
Walk through the room, past the crate, let him see you but do not give any attention, no eye contact, no words. Sing to yourself or hum, but nothing towards the pup. Act busy on something else for a minute or so and leave the room again. It will help show him you are there but not responsive to his attention seeking.
Other good