Toddler not sleeping in his own bed

[Sashatodd]

Hi all,
I have a 2 1/2 year old son and has always been really good at sleeping in his own room, we keep his bedroom door shut and until now his never got out of bed in the night or mornings when we wakes up he waits for me to come get him. The other day his room was really hot so we left his bedroom door open and during the night he came out his room and into ours and refused to go back to his bedroom so he slept in with us. Last night just as I was getting into bed he done the same again opening his bedroom door and running into my room crying. I tried to pick him up and tell him he needs to sleep in his own bed but as I got to his room he just creamed and grabbed hold of me. Can anyone please give me some advise i really have no idea how to deal with this phase i don't want to put a gate up his door as he will just scream all night but at the same time I dont want him in my bed he needs to be in his own. Thanks

Possibly scared of the dark, he's about the right age for it.

Have you got a lamp you could keep on all night in his room?

Yea his had a night light in his room for a while now as he started to get nightmares and wake up crying in the night about a year ago. To be honest I've never really let him sleep in my bed as I never wanted to get into that habit so I think because we let him do it once his realised that if he comes out of his room he can sleep with us. Thankfully the past 2 nights his not come into our room just when he wakes up in the morning he opens his door and comes into us which is fine.