Kathy:  I wonder if this is the kind of drop off that was seen by Dewey et
al. in their study on growth of breastfed babies.  I recall that there was
rapid growth in the first 3 months then a decrease, followed by continued
growth in a lower percentile.  How is the length and head circumference?
They should be continuing on the previous track even if the weight has
dropped off.
I think the mother's approach of nursing more often is a good one, and may
have more effect than trying to increase the fat in her own diet.  I don't
know if distractability is another factor with this baby, but 4-month-olds
are notorious for being more interested in everything else around them
than nursing, so it might help if she nursed him in a quiet, darkened room
for a while.  Hope this helps, Alicia Dermer, MD, IBCLC.