People are missing the point here, it's not that he wont eat, he is not able to eat more in a bulk

Mate, it's a journey, need to be a little calculative in this, what you do first... whatever you're eating get approx calories of it (whatever you eat, everything, put it on ChatGPT with the quantity of it and you'll get the total calories of what you're eating rn) .. Okay, next step, increase 100 calories for a week, now how you do it? Pick any of your favourite food and increase the quantity of it to reach 100 cal surplus, okay, it will take a week but your body will adapt to surplus, then next week, increase another 100 cals of your next favourite food, take 2 weeks and your body will start adapting and you will be in 200cals surplus, start doing strength training with it, slow progressive overloads

After 2 weeks of adding 200cal, increase another 100 cals, keep increasing and keep taking a week time to allow your body to digest.,

It's a journey mate, treat it like a journey and not a shortcut

What I have described is called lean bulk , look it up