Sounds exhausting having to sit in his room several times per night!
Honestly I think it's just time to have a chat with him, be straight forward about it and be clear on what is going to happen.
Nightmares and monsters are horrible and truly terrifying BUT they are not real. You've given him the due sympathy for the bad dreams and fears he had but IMO if you continue to sit in his room you may be feeding the fear by agreeing that Mummy needs to keep guard. You have already said he hasn't had this nightmare again but rather it is fear of having the nightmare, he needs you to guide him away from that fear by reaffirming that he is safe to go to sleep alone.That's not to say I think you have done anything "wrong," I don't, I think you did absolutely the right thing in the first instance and going forward, BUT now it's time to stop.
I would make time in the day for a proper chat, tell him there is something we need to talk about. Then cover a few different things:
1. Monsters are not real. The only people in the house are (name them, Mummy, Daddy...sibs, pets) and Mummy and Daddy are ALWAYS looking out for him, no matter what.
2. Our brains can make things up. Dreams about monsters are made in the same place as dreams about ice cream. You don't want to put "blame" on him by saying it's his own fault and his own brain making this stuff up but I do think it is also possible to explain how the brain works a little and that the existence of good and bad in his dreams is from his own creation. The monsters are no more real than the ice cream.
3. Tell him he hasn't had the nightmares for some time and it is now time for Mummy to get some proper sleep. Mummy can't play properly or be the best Mummy if she is awake lots of times in the night and you want to be the best Mummy you can be. You need your sleep and he needs his sleep - tell him that. Tell him how important sleep is for growing, developing and mending our bodies.
4. Repeat he is safe
5 Repeat Mummy and Daddy are ALWAYS looking out for him and taking care of him.
6 Introduce what is going to happen - if/when he wakes in the night he is to snuggle down, remember Mummy loves him, grab his lovey/cuddly, think of happy thoughts and go back to sleep. This is his job now, to go back to sleep.
7. Tell him what will happen if he calls you - you will respond but you are not going to sit in his room until he falls to sleep.
8 Be clear what you expect from him - a good nights sleep for all the family, we all need our sleep. Turn over and go back to sleep.
Then only go in once per night, cuddle, kiss, everything is fine, go to sleep. Leave the room.
If he is very upset stay a little before leaving the room, but do not sit and do not agree to sitting whilst he falls to sleep.
From there verbal reassurance from outside the room or WI/WO - whichever you feel is more suitable. Use your usual key phrase if you have one, otherwise "everything is okay, go to sleep" is fine.
I think really at this point you have to show him the way and you can. He can be back on track soon.
I've had phases of multiple call backs etc, usually around the birthday and half birthday. I'm having a phase now with my almost 6yo and have told him now that he gets to choose how I spend my half hour at BT, I can either cuddle in bed and read his book with him (which is what we usually do part of wind down) OR he can call me back in for half an hour and I'll go up and down the stairs or sit outside his room telling him to go to sleep - his choice. I've told him what I am not prepared to do is spend half an hour reading books and cuddling and then a further half hour up and down the stairs. He went to sleep right off one night after making his choice for me to read, but the next night started the call backs again. Tonight I am not reading with him, he will read alone, then I will go up to kiss him goodnight and tell him I'm available for call backs for half an hour. The truth is I will ALWAYS respond to him, but honestly when I've been up and down the stairs 6 times my response is unlikely to be what he wants, yk?
I know it's a phase.
Hope this helps.