Re the 5 1/2 mo old with no wt gain last 6 wks:
Does the baby sleep with mom and nurse at night?
I find lots of babies who enter a more active, exploratory stage make up for
not nursing alot during daytime by nursing at alot at night.
This 4 - 5 mo stage is one of increased distractibility.  This is true, too,
of 9 - 12 mo olds starting to cruise, walk, explore and have no time for
nursing during day.
Teething can be occurring too and babe needs some pain relief here.
The mom is probably already busy with the 3.5 yr old too and may find it hard
to sit, lie quietly with baby during day.
Breast massage during nursing may help increase fat content too.
I am not opposed to starting solids if baby shows an interest, e.g. ripe
banana or baby cereals.
Make sure to rule out illness, e.g. yeast, chronic ear, or UTI.  The MD will
have to check him well.
Hope any or all of this helps.

Laurie wheeler, rnc, mn, ibclc
violet la