Just to add on, here is an article which listed some signs that baby may not be drinking enough:
http://www.babycenter.com/0_how-to-tell-whether-your-babys-getting-enough-breast-milk_617.bc?page=2
If your baby is gaining weight, and diaper output is alright, there should be no need to worry about the latching and the amount he is drinking. Like what Vanessa mentioned, it is very likely that your baby is nursing for comfort.
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