This is pretty normal for babies to start eating more efficient at around 6-8 weeks. They eat faster and drain your breast better than any pump.
To see if your lo is having enough milk, look for output, not input. He shouls have enough wet and dirty diapers (norm is 6-7 wet per day and 2-3 dirty per day). Is he gaining weight? In second week babies start gaining 1 ounce per day till 2 months and 0.5 ounce per day till 6 months.
Is he happy, content, goes longer between feeds, is playful and allert?
If yes, you don't need to worry.