I would take him back to his room, tuck him into bed, sit on the stool and try again. I would not speak to him or have a lot of interaction with him, but reinforce that you are there BUT he needs to be in his bed. Do not stay only til you hope he sleeps, but until he IS asleep.
Somewhere around 3 my DD developed a fear of the dark. Our compromise is that if she stays in her bed, I will leave her big light on until I go to sleep. She knows if she wakes up in the night that her nightlight will be on and that we are just next door if she needs us. So I would try to find out what his problem is and if a solution like that might help. I lay in DD's bed with her one night and told her to look through her door and asked her what she saw. She saw our room. I reminded her that it's not very far away at all. She also has a bear she always sleeps with to help her feel safe.
She's 4 now and sleeps very well in her own bed, without us.