Feeding

How old was your little one when you started feeding on demand instead of every 2-3hrs? My LO is almost 6 weeks old and from time to time will go 4 hrs during the night without waking up to feed. His pediatrician insisted I wake him up because they dont want him to go 4 hrs without feeding. Mind you, he was born over 10lbs and was already a little over 12lbs at one month old..

Also, lately he will fall asleep during feeding and will only finish 3oz. He usually gets 4oz.

Any idea what could be going on??