Just like Roll Farms said, don't worry about it too much, just work him up to those 20 oz bottles.
we have one little piggy...I mean goat...who would be content if I handed her the gallon jug of milk and a straw, which I don't...but it would make her day...
The other one still barely wants 16 full ounces most times at 6 weeks old now...but she is CONSTANTLY munching on grass and weeds and feed and hay already...and growing well enough that I don't worry too much. She never really yet has taken more than maybe 4 full 20 ounce bottles...but she's growing like her sister
I just let them work up to what they can handle at that 20ounce mark - some get there in like a week I swear and others never do....they make all these handy goat hand books and internet websites with instructions...and then the darn goats DO NOT follow the rules!
Hoover was a piggy like that, lol. He was getting three 20 bottles a day, and would have a round little belly when he was thru, but still scream like I only let him lick the nipple, at least until he decided to wean himself.