I agree on the app-- love Presence! You can talk to them through screen too unless you think that will upset him more.
I usually leave a fan and/or a TV on when I leave Jackson in an apartment -- I live in a home but I dogsit often for two small dogs in an apartment and on the rare chance I have to leave him to run out quickly, that's what I do. He, too, is not used to apartment noises so can bark for first few days at people coming and going outside the door.
But the fans and TV can drown out some noise. You can also work with him one day when you can maybe stay home. Make sure to give him treats when he remains quiet. Teach him a 'quiet' command. I think
http://www.youtube.com/zakgeorge21 has videos on this.