As far as the junk food; toddlers need surprisingly little food volume and variety isn't a concern yet. As long as he's not underweight (if he is, this is a medical issue instead) then I would offer a selection of foods and stand back.
If a baby's on the low end of the scale, don't forget the scale belongs to the AMA and has to do with statistics, rather than with the health or realities of individual humans. ?There can't be an average without some people at the extremes, and some are naturally at the extremes from genetics, and they are at their OWN "normal" (midrange for their own body's development, as their body has no idea about other organisms of the same age).
I'm guessing by "underweight" maybe the writer meant unhealthily scrawny or dangerously thin. ?
Holly seems scrawny to me. ?She always has. ?But she's strong and active and so I just don't worry about her bones showing. ?I'm sure it will pass sooner than she'd like it to (given the prevailing prejudices in the land in which she lives).