Very picky eater

My 12 year old son has always been VERY fussy when it comes to food and it’s a nightmare! He won’t touch cheese unless it’s on pizza. He doesn’t like margarita pizza and only eats pepperoni pizza but he takes the pepperoni off. I don’t understand. He’s quite tall, maybe around 5 foot 3. I’ve been trying to help him put on some weight as he’s not even 40kg yet. People have said to me that he’s fine to be that weight, but he is not eating a healthy diet and will only eat small amounts. It’s also really hard to get him to try new foods. At the moment he will eat plain pasta with black olives and no sauce, fish fingers, chicken nuggets/tenders, cereal, berries, yogurts, jam on toast, salmon and some other things. He won’t eat frozen fat chips and asks for thin ones. He is very unpredictable sometimes as he won’t eat chicken nuggets if they are from a different shop or brand because they look and taste different. The other day I made baby potatoes in the air fryer and chicken. I made my daughter homemade chips because she asked for them. My son refused to eat the potatoes because they were a “different colour” than they normally are when I cook them. I told him things won’t always be the same all the time. I put them back in the air fryer and cooked them a bit more. He said I ruined them and he wasn’t going to eat them and he wishes I made him chips because they taste better even though it’s basically the same thing. I told him he couldn’t leave until he ate at least 2 potatoes or one more piece of chicken instead or I will take away the PlayStation. He refused. I don’t know what to do anymore. I cook usually cook healthy meals for me and my children and I don’t like to see my hard work go to waste. It’s fine if he doesn’t like certain things but he is very picky and will say no to a lot of things or say they look disgusting. He will even cry or whine when I say he has to eat what I served him. He’s doing really well in school but spends hours on the games past midnight talking to his friends. I have fibromyalgia so it’s difficult. I told him he is lucky to even have food and some kids in other countries don’t. He will drink so much cola and orange juice and eats so much rubbish like crisps and sweets and leaves millions of cans, crisp bags and wrappers around his room. I find myself going all the way Costco to buy him Kimchi. We don’t have it because I couldn’t find it won’t be buying it for a while because it’s so unhealthy. My son says eating unhealthy food like crisps and McDonald’s is gonna make him bigger because he needs fat but I told him he needs to eat a healthy diet. How can I help my kid? Please help.