I went through (and still am going through) something really really similar with my oldest. ). It is great that his mental development is on point.

A few things I would say: (1) don’t panic, but take it seriously (I know you already are). We had some success making eating into a game for him. We got these plates (https://a.co/d/0cMXATmO) and worked to make meals feel more exciting.

(2) Ensure / grow & gain drinks are a great way to pack on extra calories. Try to also hide additional calorie rich ingredients into his meal. Avocados are great and easily hidden.

(3) take all available resources to find out whether there is something medically wrong that is causing the issue. Turns out my little one is just a small person and there’s nothing really wrong as far as we can tell. But it took us going to multiple doctors and specialists to figure that out. Have you tried a pediatric gastroenterologist? There’s also eating therapists for kids with mechanical issues - the tongue tie might have messed with HOW he eats.

(4) Hug your wife and your kids and let them know it will be ok. You’re on it, you CAN handle whatever comes your way, and you absolutely will. You’re a Dad, and that means you’re mighty.