4.5 year old having separation anxiety at night

So on and off the the last few weeks my little boy wants me to stay in the room with him at bedtime until he falls asleep. The past few nights however he has been getting really upset if I want to leave the bedroom and is awake until at least 9.30pm when he would normally be asleep by 8.30pm and waking during the night. I'm at my wits end trying to figure out what could be wrong with him. He says nothing is wrong and nothing happened that he just wants me to stay with him. My husband and I had an argument last weekend which we don't think he heard but now I'm doubting that and he stayed with my mother-in-law last Friday night and ended up sleeping in her bed cos he was upset and she didn't want to let him sleep in the other room (she doesn't like seeing him upset at all) Does anyone have any ideas as to how I can help my son at nighttime? Sorry the post is all over the place!!