Just a thought- I'm not sure of this so I'm just throwing it out there-
maybe his tummy got used to not getting as much when he was sick, and now he's trying to get back to where he should be. I imagine it would take a little time.
Do you have a set schedule during the day- as far as always wake @
same time, and always go to bed around the same time in the evening, and have pretty similar schedule throughout the day?
Also, how is the daytime sleep? That can have a big affect on nighttime as well.
But yes, they will make up for nutrition during the night if they don't take enough in the day.
Hang in there!