OMG this thread is terrifying - so many people talking rubbish about healthy eating it's untrue.
The facts we have are: boy is a fussy eater, he is over-weight and he loves sweet tastes.
I think the advice you have been given about cutting fats and upping carbs is out of date - the carbs are just complex sugars and will b driving his desire for more sweet tastes. It's sugar that is his real enemy. Fats (in moderation) will help him feel full.
I also wondered if you have had advice on portion sizes? You are clearly very anxious about making sure he eats enough because he is fussy but at the same time he is overweight. He may simply be OK eating less and being left until he feels really hungry - he doesn't sound like some of the other posters fussy eaters who are underweight.
So sugar-free squash is better for him than normal squash as fewer calories (ignore the aspartame madness) but it still encourages him to think all drinks should taste sweet. Juice is acidic and v high in sugar - a glass of orange juice contains the sugar of 8 oranges, when would you eat 8 oranges at once? So ultimately water would be best.
Cereal isn't great and Nesquik cereal definitely isn't as it's so sugary but you are in a bind as he is so fussy. You could keep it for treats at a weekend and have a less sugary cereal the rest of the time. You should also weigh the portions according to the recommendations on the box - most self-poured portions of cereal are much bigger than they suggest, often 2 or 3 times bigger.
Skimmed milk won't be cutting down his calories much - semi-skimmed or even whole milk would have more vitamins and possibly help him feel full.
Ideally he should see that everyone in the family is having much the same as him ie the same milk, drinks etc so he isn't a special case - it will be much easier to get him to drink water if that's what everyone drinks for example.