I read your question earlier and have noodled on it for a few hours.
I like what other posters have said. I wanted to add a couple things:
First, you are very wise to recognize that you are falling into a pattern that will not work long-term. I read that 'when your are together in the evenings' and am assuming (perhaps incorrectly?) that you are a working mother. I think working parents are even more tempted to fall into the trap of meeting their children's demands because we don't see them all day, and then want our few hours with them to be pleasant, so I get where you are coming from. And you are smart to want to change things now, because I've seen what this looks like when children are older and it just gets uglier.
If it were me, I would offer a solid dinner. After dinner, if he's still hungry, offer ONE (only one!:) ) plate with nutritious foods that you know he doesn't hate. He doesn't have to love them, he just doesn't hate them. Almonds, cheese slices, apples, pears, whole grain crackers,carrots, celery, hard-boiled eggs-- whatever works for you. And then, let your mantra be "This is what I have for you right now." When he whines and asks for other things, you can 'write a list of ideas for other times' but keep reminding him that "this is what I have for you right now", and be firm with it. He's going to likely be upset, because he'll want you to change back to his way of doing things, but you are all done with him being in charge. You are the mom, you are in charge, and you DO know what's best for him, whether he agrees with you or not.
I have a blogpost I wrote a while ago which has some of my favorite 'pat phrases' I use with kids when they get stuck on Whine Mode. Here's the link:
http://skyteahouse.blogspot.com/2010/09/what-to-do-with-w...
Last of all, you suggested a plan for discipline. I really like JoAnne Nordling's "Taking Charge: Loving Discipline Which Works at Home and At School". This book helps parents not just find discipline techniques, but to uncover what's underneath the behaviors we are seeing that we don't like, and then how to address each realm of misbehavior. (There are four). Nordling offers a perspective of balance, empathy for the parent, and a lot of common-sense not-punitive corrections for misbehaviors. I'm a preschool teacher and many of my colleagues also swear by this book. It goes so much deeper than "when they do this, do that" to help parents really get a glimpse into the child's reality. This makes US feel better, too, when we have to discipline, and helps us to become more comfortable and confident in being authoratative parents. This book has had consistently positive user reviews and is required reading for several preschool parent communities.
It does sound, too, like much of the whining is an Attention-Getting behavior, and another way to meet that need for your son is to tell him what you 'are' available to do. "I'm not ready to get out the marble-run right now. If you want to do something with me, let's find a book to read." (or whatever else you want to do with him at that moment) When we acknowledge what's under their nagging and whining, they get that empathetic connection and can sometimes move on.
At other times, too, if you are busy and need to take care of yourself, you can let him know what you need to do now, and when you can help him with what he needs. (there's more on this in my blogpost as well.)
This is long enough and you've gotten some great advice. If you need to tell yourself something, how about "I'm the Mom, and I do know best, even if he doesn't like it! If he was capable of being in charge, he would have hatched out of an egg and walked away!" ;)
H.