Hospitality is hard, isn't it? I think you went to a reasonable length when you made him a breakfast and then gave him the apple as an extra. The food was good, it was good for him, he ate a bit of it...I would call that a success. Would I go far and above that next time? No. When we have other people's children over, I have raw fruit and veg, hummus, nuts, dried fruit, cheese, applesauce, pickles, olives and cinnamon toast or nutella as my go to foods. I usually make up a platter daily for my kiddos with all these items, and if other kids are over they can have some too. If they don't like any of that...then they can wait for mom. I've made a reasonable effort to make sure there is something for them to eat...and remember, a person only needs to eat a portion of food the size of their fist to be satisfied...so in reality, they don't need to be stuffed to the gills with their favorite food for you to have been a good host. Most children love warm tea with honey and toast usually.
I would not go out and buy a special food that I would not normally feed to my own children, for one of their friends. If when they are teens and they have a best friend that becomes part of our family regularly, then I would find an acceptable food that the friend likes and that I wouldn't mind having regularly and I would add it to our lives. Until then, what I normally have is good food, whole foods, and if the friend's turn their noses up at it, that is their choice and I have not failed them. You haven't either. Reasonable effort, a few bites...success. Fistful portions.