I'm not an expert. But just to share with you my own experience lah:
For the first two months, my BB just eats and eats.
Then, when he got older (3 months onwards), he's become very distracted. Wanted to play, play, play and play all the time and sometimes he would cry when I let him drink milk. I think he drank less too, but because I use direct BF most of the time, I can't measure the exact volume. Anyways, if weight gain okay, no problem lah! You can't force BB to eat, right?
By the way, is your BB a bit ill? Maybe he'll eat more when he recovers wor.
Good luck!
P.S. Where did you buy the Santa Claus clothing for BB?