I only changed DD up to the next flow when she started becoming frustrated while eating and it seemed to take ages for her to finish her bottles. She moved up to medium flows at 4 months and then to fast flows at 8 months. These days she's a very efficient eater and usually drains an 8 oz. bottle in less than 10 minutes, sometimes it takes only 5 minutes! If your DS is feeding efficiently on the #2's then I don't see any reason to change. If that changes, he starts dawdling and seems to be getting frustrated with the flow, then I'd try the #3's again. It took DD a few days to adjust to the faster flow when I changed her up - I usually did it on a Friday and by the Sunday she'd be eating normally again.