Maybe you are going about this too fast. I wouldn't do a complete diet overhaul just yet -- just introduce one healthy new food at a time to replace one of his unhealthy favorites.
If you are now serving him whole grains and more raw vegetables and fruits (when you weren't so much before), I would definitely caution you not to give him too much at once. It can screw up his digestive system and cause him to be constipated. Give him small servings at first, once or twice a day, and then just add on another serving or gradually increase the portion size maybe every 2 or 3 days or so.
As for getting him used to drinking water instead of juice, I would not take the juice away completely but would gradually add more and more water to it so that it eventually becomes more water than juice, and then just water.
One trick I used to get my son to try new foods when he was that young (he had a lot of food aversions) was the "smell, kiss, bite trick". It goes like this:
* Put a little bit of the food on a fork and request that your son to smell it and make sure that you are using your fun, playful voice when you do this. My son always said, "yucky!" after he smelled the offending food.
* Next, tell him to kiss it. This will give him a chance to get a sense of the texture and taste of the food on his lips. My son always said, "yucky!" after this step as well.
* Next up, you then tell him to taste the food. Just a tiny bite will do. At this point, my son would usually exclaim, "Mmmmm... that's delicious!" before devouring the new food item. If he does give me a third and final "yucky!" instead, then he is off the hook for having to eat the new food item for that meal and that meal only.
It takes a number of different tries until children become accustomed to new food tastes -- can be anywhere between 9 or 12 times. Don't give up but don't push too hard and make it into a food war either. Small changes over several weeks time is better than a full-on total change in his diet. Not only is that chaotic and confusing to him but it can cause him a lot of gastrointestinal distress if you don't time things right.
I hope this helps. I think it's great that you want to teach your son good healthy eating habits. He'll be so much better off for it.
I wish you well.