My son is 4 now and he went through a phase similar to your sons. He would not let me go anywhere without him and there were always monsters in his room. I don't know why this was but he grew out of it . I know it is frustrating but hang in there he should grow out of it. Make sure there are not any scary shows that he is watching as they can make things worse. I took away anything that I thought might even be a little bit scary during that time and it seemed to help. Make sure that he knows where you are when you leave a room and try leaving the hall light on for extra light. Find out where he thinks the monsters are and show him that there are not monsters in his room. I found that stuffed animals really helped my son with this. He was allowed to sleep with as many as he wanted because they made him feel safe. Maybe let him buy a special stuffed animal that will protect him. I know this sounds strange but I told my son one night that his Dumbo ate monsters and he bought it and went right to sleep. I don't condone lying to your kids and he knows there are no monsters but I think the idea just made him feel safer some how.
Hope this helps
Kaylee55 was right with the stuffed animals, it worked with mine too. My oldest son was 2 1/2 when he started being afraid of monsters and it was because the next door neighbor told him he had monsters in his closet. Mind you it was a full grown man, (my son was playing with his son) Try to find out if an older kid is scaring him or worse an adult!!
Thank you so much. Him and his sister share a room because they are 16 months apart. She started being scared of the monsters which he started copying. He slept in her bed (after asking her to be a big sister and protecting him) for a couple of nights. He already sleeps with 3 stuffed animals. We shoo the monsters out and yell at them to get them out. He laughs and tries to help out. He is doing much better. However, he is still waking up 2-3 times a night and I can't figure out why.
I'll admit, we did the "super-nanny" method. Our son was getting up and getting in our bed in the middle of the night (would run in and slam the door as if afraid). Before we tried it all, fish tank too! What finally worked was putting him back in bed each time he got up (even if scared, belly ache, hungry, thirsty - we heard it all). To help with the fright my husband and I took turned actually sleeping on the floor right outside his room for about a week until he was comfortable staying in his room all night. Each time he got up we were right there to say "go back to bed, your fine". He wasn't scared b/c we were right there. It went from 3-4 times getting up to one time after a week. And even the one time he would go back without a fight. It was all about making him comfortable staying in is own room all night. Sleeping on the floor outside his door did it!