Hi,
I think it is great that he was willing to eat the stew, try the tomato and that you included his favorite as an enticement.
This is a difficult problem, especially if parents' have defaulted to the child's diet of choice. Our younger child was a picky eater. Here is what we did to help her (and ourselves!). We talked to her about it at a time when we were not eating or getting ready to eat. No battles at the dinner table--it only makes it worse. We told her that it was important for her to develop a taste for a lot of different foods. We also told her that we understood that she thought that she did not like most foods and only liked noodles, bread and such. We explained to her that if she did not try anything, she would never know if she liked it or not. She was also told that she might not like broccoli, for example, at five years old, but that she might find out she liked it when she was six. How would she know if she did not try it several times in her life? So we told her this was our new rule: she would be expected to try whatever was served. We would not force her to eat. However, there would be no special items made for her. She could choose to eat or not to eat. Of course, we made sure that she had those things she really liked to eat many times during the week. This was highly effective. She is 14 now and generally eats what we serve with some exceptions.
The other suggestion I have is to get him involved in meal planning and food shopping. At first, offer two or three choices. What would you like to have for dinner tomorrow: spaghetti, meatloaf or chicken? Then let him choose the side dish or dessert.
I grew up with an extremely picky brother. My mom made whatever he wanted and he never had to eat what the family ate. He is still very picky and difficult to please. It is embarrassing to go to a restaurant with him because he almost tells the waiter how to cook the food (stay home!). His kids are also very picky, to an extreme. Nip it in the bud now! Good luck...he is fortunate to have parents who want him to be healthy and happy.