At 21 months hunger at night will only be because he "learnt" it, infact at that age, he could go probably 36 hours without needing food, just if he gets fed at night his body is used to it & asks for it.
21 months was when my ds#1 got his eye teeth (big ouch) as I was due anyday with ds#2 & not liking being up 3-4 times a night.
I gave pain meds, but seriously it didn't seem enough... he was still upset & not sleeping well.
Add in that it could be a 'wave' of separation anxiety, I'm guessing that it is a time thing & as long as you are consistent things will go back to normal