I agree...while I like the snuggles, I'm not a huge fan of sleeping in the same bed as my kids.
Do you think he's truly scared or has it just become a battle of wills?
If you think he's just doing it because he can:
Since he shares a room with his older brother, how about they have a camping party for a few nights? Make a 'tent' in their room and let them both sleep in it for a while, even if it's on the floor. IF he's excited about this idea tell him he can sleep in his tent OR sleep with mommy, but if he sleeps with mommy the tent gets put away. Could maybe have a tiny flashlight for the tent too to help him shine away the nightmares. Would really only work if he's super excited about the campout, otherwise the plan will horribly backfire lol. Maybe the excitement of something new will help him forget about his fears of his bedroom.
If you think he's truly scared of his room:
Maybe put a sleeping bag next to your bed. Tell him he can either sleep in his big boy bed or in the sleeping bag, but not with mommy. But in the meantime, maybe let him be near so you can help 'in-case' he has a nightmare, but not so much that he gets used to sleeping in your bed. At some point it's just going to have to be non-optional to sleep in his own bed...but I would make sure he's over his fears before trying to push him into his own bed, maybe give it a week or so.
Just some ideas, looks like you have some other good ones below too...hope you guys find something that works!