I just tuned back in to lactnet after being offline for a week+ so forgive me
if this has already been discussed but I received a call once years ago when
I was first starting to do breastfeeding counselling ( before doing formal
lactation consulting) just like this.  The baby refused any other foods, was
fussy all the time, and was not gaining weight any more, though she had
gained well for about 10 months.  She had "milk anemia", specifying anemia in
a baby who is only taking breastmilk.  Have you already considered this
possibility?
Betsy Wall, IBCLC