Yes, Disney has always been great about such things. In some cases, he may be able to order 'adult' portions of kid's menu items. Most kids' menus have items like plain grilled chicken, grilled fish, or macaroni and cheese (though at some places the mac and cheese is now the Kraft kind, not the real homemade kind.) If you know he loves Mac and cheese, for example or pizza - you coudl simply phrase your request that way -ask if he can have a specific dish they already make. This is particualrly easy if you have
DDP.
I also think buffets may be great because he can get plenty of plain food - AND - maybe be ALSO tempted into tasting something new. At buffets, he can take a tiny sample of anything, even put it on it's own plate. He doesn't have to commit to a full portion -and he can see the food before he serves himself - so he can control it all.
A favorite in our family has actually been Boma, because everything is so colorful. I've been there with several picky eaters and had great luck with them chooing to try a few new things. Not anyone telling them they had to do so - just on their own.With a limited eater, any new food is big victory. And the adventurous eaters will likely love it too!
Beyond that, Pepper Market would likely be a great place too (at Coronado.)That is almost like a buffet, and you get part tabel service for QS credits if you have DDP. Especially if they happen to be staying there.
Personally, I'd also let him pick at least one place. He can look online at the menu of almost any TS place, and decide what he wants to try. Sometimes that is best way to handle a picky eater. If he has control over at least one choice, then he may feel happier about the whole week. Without knowign the person; it may be that he likes only plain 'American; food,or he might like 'plain' sushi...
Really though, we've found every place has some plain dishes or things that can be made plain (skip the sauce). I've been with severa picky eaters.
If all else fails, they could always eat TS- then grab him a quick serve meal someplace nearby after. I REALLY dont't think it will come down to that at WDW, but there's no shortage of places to grab quick food.
Another trick with a picky eater is to keep something on hand that he knows he likes - such as a granola bars, fresh fruit. Ideally something filling, portable, and healthy. IF they won't have a car, there are places that deliver groceries. IF they have a car, they could do a short grocery store run. Just to have a few back ups on hand. Fifteen year olds tend to be big eaters!
If that still is not enough - try calling special needs dining services. They can happily tell you exactly what every place is able to do. I KNOW every place has vegetarian options, heart healthy options (which is basically the same as what you want)- even if they aren't on the regular menu, or on the buffet line. If they have salmon on the line with a sauce - they can get you a plain portion if you ask.