Sugar is pretty much always bad and anything obviously sugary should be kept to a minimum. Particularly when it comes to your DS, be aware of high levels of sugar in juice and dried fruits, more for the benefit for his teeth than general health I think.
I think the best way to keep salt intake down is to eat fresh food prepared yourself and try not to add any/too much. Anything processed (with the exception of most pre-prepared baby food) generally has pretty high levels of salt and so does food served in restaurants, whether its KFC or Gordon Ramsay!
The following is copied from a website...
BAD FATS
These include saturated and trans fats. Most children and adults in Britain are eating too much saturated fat. They are found in food like:
X Full fat dairy products such as cheese, butter, full fat milk and yoghurt
X Fatty red meat such as sausages, salami and luncheon meat
X Processed and fried food such as pies, pastries, pizzas and takeaways
X Cakes, biscuits and pastries
X Snacks such as crisps and nibbles
X Confectionary such as chocolate
Trans fats aren't always listed on food labels, but they are mostly found in foods containing hydrogenated oils. Hydrogenated fats are used in processed foods like puddings, biscuits and other snacks. Look for the word 'hydrogenated' on the label and try to limit these foods
Good fats = Polynsaturated fats found in sunflower seeds, corn, soya beans and walnuts and their oils. Vegetables, fruits, nuts and grains are also good sources
Omega 3 oils found in oily fish like mackerel, pilchards, kippers, herring and fresh tuna. Everyone should eat at least two portions per week.
Mono unsaturated found in olive oil, rapeseed oil and some nuts like walnuts, peanuts and almonds
Plant sterols and stanols help lower cholesterol levels. Found in some margarines, milk and yoghurt products to which they have been added. Only recommended for adults.