My little one sleeps in a play pen downstairs by herself. In the play pen is her bed a kennel with water, and a pee pad. I usually tuck her into bed and have a ritual where I tell her to go to sleep and cover her in a blanket, put toys around her and turn on a night light and then go upstairs. I'm doing this until she's well trained well enough, then she'll be free to roam around the house.
It was hard at first to listen to her cry downstairs, but her crying became less and less and she got used to it. I remember I got so little sleep the first two weeks. But she's 4 months and perfectly fine. She knows she'll see me when the sun rises.
and she knows she's not going to be alone for long. They are smart and will figure it out. Just don't keep picking him up every-time he cries.
I realized sometimes Lexi cries but she's just trying to get her own way. So don't make your little guy take advantage of you. He'll get over it
Also I hear if you put a warm bottle beside them they'll fall asleep better. He's probably look for warmth from his litter.