Jenny B your grandson and his fish and chips is a perfect example. Children make associations on things instantly. The minute he was introduced to breaded fish at an early age, he made the association, decided he liked it, and now nothing else is fish to him. Same with the chips, (and possibly potato crisps as well) nothing else is potato. He also sounds to be particularly smell oriented, while many are more visual. The trick is to avoid reenforcing it too much.
I totally agree with Sidevalve, today's parents (and of course grandparents) give in to a childs whims and moods far too much for their own good. I raised my kids at the same time as my sister-in-law and in very close to the same area. The result... none of my kids are picky eaters and both of hers are to this day. It came from them growing up pushing away entire plates of food, never eating a single leftover, and allowing them to accept or reject whatever they wanted.
The most common explanation I've heard is "Well they're only children for such a short time". Well, that's right, and they're adults for a heck of a lot longer. And even adults who grew up fussy eaters don't really like being that way now, it's inconvenient to their lifestyle, such as when they go out to eat for example.
I also believe this highly selective eating process when someone is young positively contributes by extension, to the "allergy craze" currently underway. And just like movies and music, some are more popular than others. And in my opinion, in both my personal life as well as in the professional kitchen, the Top Hits are currently MSG, gluten and garlic. I'm not saying these allergies aren't real, but I am saying it's very often "popular" to have them-- just as being "vegan" is another currently trendy thing.