Since going paleo, my skinny husband has become even thinner (sometimes down to 112 lbs at 5'5"! Yikes!) He's pushing himself to eat more protein and do more bodyweight work to get muscles going. I suggested that he also up his carbs using starchy sources like sweet potato after workouts.
He used to eat and drink a lot of sugary stuff which is how he was keeping weight on before, I guess. Docs have tested his thyroid and it's supposedly normal.
Any better tips for him beyond "eat more"?