First of all Welcome!
Your post reminded me of the baby of a friend of mine several years
ago.  Baby was a premie, but even after reaching gest. age of 40 weeks
and beyond, he was a slow gainer.  You could have even called him
scrawny.  Even the staunchest breastfeeding supporters among us were a
little worried.  Fortunately, she had a most supportive pediatrician who
kept saying that developmentally he was fine and she shouldn't worry.
We lost touch for a couple years and when I saw the boy at age 4 I was
amazed.  He was still thin, but smart as a whip.  One possibility is
that mom has Marfan's syndrome one characteristic of which is a long,
lanky body.  Her son has some of the same characteristics and may have
unfortunately inherited the syndrome.  This might have been one factor.

You did mention that in your case the baby did seem to be nursing a
great deal.  One possibility that comes to mind is retained placental
fragments.  Has her bleeding been within normal parameters?  If she has
had more than normal, this might be an explanation.  Is he having at
least 6-8 wet diapers per day?  Some babies at 8 weeks will skip days on
the dirty ones.  It may not be as reliable a measure as in the first
month.  However, if he is having several stools a day (not just stains,
but some real volume) it would indicate he is getting a good volume.
Are there other people in the family that either gained slowly as
infants or are smaller size as adults?  If so, it may be a familial
trait.

Weight is only one indicator.  If he is healthy in every other was and
is showing normal development for his age, he's probably doing what's
right for him.  If not, mom may want to consider trying to increase her
supply with fenugreek, More Milk Plus, or Domperidone.
(Archives should have lots of info on these.)

Good luck with this nursing couple.
Winnie Mading RNC, IBCLC

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