By UK standards, he doesn't sound fussy at all and you sound very reasonable, and them rude, so I don't think you have anything to worry about.
Personally though, living in a different country with different food cultures - going to get an alternative for your child because they don't like one part of the provided meal would be considered rude. A child choosing to not eat something is fine, we all like and dislike things, but the parent then getting their own alternative wouldn't be considered good parenting.
The reason why is that children's tastes change and develop, so what they dislike now, won't be a dislike in the future. Your son doesn't like gravy now, and that is fine... but if gravy is part of that meal/dish, it would be provided every time, and everyone else would eat it, and every time he would be offered it. The narrative given to the child is that its ok to not like it now, but they will one day when they are a bit older. Chances are, at various points he would try, and at some stage in his childhood he will suddenly start to like gravy.
What I see in the U.K. is that disliking lots of foods is normalized, so instead of children growing up believing that they will eventually like things, they are told that it normal to dislike so many things. They are then provided alternatives. So the dislike becomes fixed - they grow up always knowing that they hate "X" food, and it is normal for people to hate foods, and it is ok for food to be swapped around to suite their dislikes.
Actually what I've seen in friends and family is that the stage of suddenly liking things can happen a lot later in the U.K. late teens, even as a student, when people almost get a reset and start to try lots of new things with new groups of people who give them subtle social "pressure" to explore and be open about eating.
I'm probably not explaining it very well.. but it isn't about forcing children to eat things, it is about a food culture where having a lot of dislikes is not the norm, and there is an expectation from everyone that you will grow out of young fussiness. I know this won't be a popular response in the U.K. but given the country's problems with childhood fussiness, healthy eating and obesity.. I'm not sure that the "correct" attitudes in the U.K. are actually working....