My son would not do the protein shakes.
#1 – We concentrate on breakfast and dinner. We found that pushing too hard about lunch was making it too stressful for him.
#2 -We had some success with 2 things: homemade “lunchables” (salami, cheese, goodie) and homemade “corndogs” (corn muffins with sliced hotdogs in them). The lunchables upgraded to mini salami-hawaiian roll sandwiches.
He is still severely underweight, but there are no issues with energy, mood, or health.