LN just wondering what your feeding routine is?
Do you
1) feed him, burp him, change nappy, feed again?
2) Or feed, burp, feed, sleep?
3)Or feed, sleep?

My babies have all done times when they fed every 2 hrs. I. Fact DS3 is doing that this week. Depending on what you are already doing for feeding, routine 1 can help make sure baby is truly full before going to sleep.
Often tiny ones will feed to sleep before their tummy is full. Other times it's just like others have said 'tiny tummies and easy digested milk= feeding often.'