All of my kids had good birth weights (8-9 lbs) and then really thinned out. DD takes the cake -- at her 15 month well visit, she was 31.5" (50th percentile) and 18 lbs, 8 oz (5th). She gained just 1 lb. since her 1 yr. visit.

She's an okay eater -- not terrible, not great. She eats oatmeal with whole milk and maple syrup for breakfast, along with Cheerios and some apple. Lunch might be half a sunbutter and jelly sandwich or lentil soup or scrambled eggs (just some of mine). She snacks on yogurt, crackers. Dinner is whatever we're having, though this is her smallest meal of the day. If she didn't eat any veggies, I'll give her a squeezie pouch (but she doesn't always finish). She also has 2 8 oz. bottles of whole milk each day.
Thoughts on what else I can give her? We haven't introduced nuts yet since DS1 has a peanut and tree but allergy.
Re: How do I help my peanut gain weight?
He's a big guy 33 inches and almost 27lbs.
If pedi wasn't concerned i wouldn't worry too much. Ask pedi what you can do to help her gain a bit more weight.
What about string cheese? LO loves it.