J.T.
Since you said he is afraid of something coming into his room, have you tried making a "Monster Be Gone" or "Monster Repellant" spray? Let him help you make the monster spray using a recipe. (You make up the recipe of course!) Find an empty sparay bottle, put in water & maybe a few drops of essential oil to give it a scent. That way he can smell it to know it is working. Let him choose where it needs to be sprayed before his bedtime/naptime. I never did this w/my daughter because I heard of it after she was older & no longer afraid. But, I've heard from MANY parents who used it that it worked for them. There were some that tried & it didn't, but it couldn't hurt to try! For some reason my daughter was afraid when she slept on her bed but wasn't afraid of "camping" on her bedroom floor. (I had to put a sheet ofer teh side of her bed so she couldn't see under her bed.) She did that for months. I don't know why it worked, but it did!
Otherwise, you are right... he is afraid, just let him sleep with your until he feels safe again. You will get more sleep w/him in bed with you rather than getting up all night taking him back to bed. This too will pass.... I promise!
Good luck in helping your little guy feel safe!
J.