I have thoughts to share with you on this topic but didn't have time to write before.
We started reducing sugar at our house about three years ago, when my kids were 4, 6, and 10. I was changing my own diet and just started changing theirs along with it. But I did it in baby steps and looked at it as a chance to educate them about food instead of just saying "you can't ever have XYZ any more." I wanted to make them part of the process, you know? It sounds like your son is very open to this approach, too.
First, instead of just taking out sugar, I looked at what we were eating overall. I focused on getting more protein and fat into each meal, then switched from refined white flour products (which acts just like sugar in the body) to complex carb/whole grain products -- like having rice pasta instead of white flour wheat pasta, for example. Then I looked at things like applesauce that they ate regularly, and stopped buying the sweetened kind (which almost always has HFCS in it) and bought only unsweetened. Eventually I started making my own applesauce, too, with no sugar. They like the homemade kind far better than any store-bought kind I've found.
At first I didn't restrict candy/sweets but I said we weren't going to eat candy by itself--only after a meal, or a snack containing protein/complex carb. So if they asked for a piece of candy, I'd say "sure, but have a cheese stick and a piece of fruit first." Soon they would come to me and say, "If I have XYZ first, can I have a piece of candy?"
Once they got used to this, we looked at cereals and started reading labels and only bought cereal that had less than 10 grams of sugar per serving. (Eventually we stopped buying cereal that had any artificial colors in it, too, as we realized that our youngest child reacts badly to dyes. But it took time to figure this out.) As time went on, and my own health was improving from all the food changes, I also started cooking real breakfasts (scrambled eggs, bacon, oatmeal, etc.) for them more often, so they weren't eating cold cereal for breakfast as much.
To make a long story short, we've made huge changes in the past three years and it has had a very positive effect on the kids. They still eat sugar sometimes, but their taste buds have changed. I do make desserts, cakes/cookies, etc. sometimes but I usually cut the amount of sweetener in half and they are very happy with that. They don't like super sweet stuff as much any more. My son was thinking recently about when we switched from sweetened to unsweetened applesauce, and how it tasted strange (tart) at first, but now they prefer it that way--and sweetened applesauce tastes nasty to them.
Another big change: this past Halloween, they picked out a dozen "special" pieces of candy (which they ate slowly over the next month) and willingly gave up the rest (literally hundreds of pieces of candy). I "bought" the candy from them in exchange for money to buy books at their school book fair. They were thrilled with this idea and have already said they want to do it again next year. They said they love trick-or-treating for the fun of getting dressed up and going around the neighborhood to everyone's houses, but they don't care about the candy. They said they don't need it; they would rather feel happy and healthy. They still have candy leftover from Valentine's Day and their Easter baskets floating around here somewhere. Every so often they have a piece but overall they're just not that interested in it.
They are very interested now in the link between health/behavior and food and will often comment on what they see other kids eat and their behavior. Recently they were playing with another kid in the neighborhood and talking about how mean and nasty he acts sometimes...and then they said, "But we notice he's constantly eating those frozen popsicles in a tube--you know, the ones that are mostly sugar water and food coloring. Plus he often eats Oreo cookies for snacks, instead of having a sandwich or a bowl of soup. So that's probably why he goes from being nice to being mean all of a sudden."
They also notice now their own reactions to food, and are starting to self-regulate more and more. It's really amazing to me that my younger two, especially, at ages 7 and 9, are doing this so adeptly. I hear them talk to each other about it and say, "you know, the last time you ate that you didn't act very nice later... I don't think that's good for you. Maybe you should choose something else." And the other one will say, "Good point. I'll have a banana, or ask mom if we can have some organic chocolate chips instead."
Okay, I'll stop now since I wrote a book. But feel free to ask questions. I think it's great that your son, at such a tender age, is recognizing that food can impact how he feels. This is huge, and can have a huge impact on his health and happiness.
Good luck, mama!