My six year old went through the same thing at around 5-5 1/2. He was the same as your son, almost to a "T" he even had to have his closet door open in his room, if it was closed he would be freaked out cause he couldn't see if there was anything in there, most kids like it closed. I told him no monsters exsisted, pointed out the obvious, and still he had this fear, he wouldn't even stay in the bathroom by himself he had to have his little sister in there with him. And then I got to the point you are at, so I told him that I was secretly a "monster slayer" and that my house was a monster free zone, no monsters could make it past our creek in front of the house, I don't think he believed me at first, but I kept telling him the same thing. Then I started to only walk him half way to the bathroom and would watch him the rest of the way, I told him if I saw any signs of the monsters that I would "save" him, of course we had no monsters they couldn't get past the creek. Slowly I would end up just having to watch him, and know he can do it on his own, it took a month maybe two but it did work. On the upside my Daughter hasn't had the same fears, cause "I am the "monster slayer" and we have no monsters.....right mommy". Thats all she needs to hear and she is good. Just a though and maybe a laugh! Good luck!