Help? My 21 month old won't sleep without screaming and crying?

I'm a first time mom and I don't know if it's a stage where your 21month old don't want to go to bed easily anymore. my son has always slept in his room by himself and by 8:30 p.m. Now he cries bloody murder as soon as I put him into his bed. I checked his temp no fever, no dirty diaper, he seems fine physically. I mean he will stop right away when he knows I'm in the room, but once I walk out he starts crying and won't stop. Does anyone have any advice?