I would first read a book on nutrition so you can get an idea of the right types of foods and ones to eat in moderation. It is the moderation bit that is the key. Cutting out fatty snacks likes crisps, biscuits, cakes, etc is a good step plus fizzy drinks. I would swap the fizzy drinks for healthy smoothies made with whole fruits not concentrated ones.
Then I would buy a low fat recipe book, look through it and see if the photos look yummy first because if you find a photo appealing more than likely you will enjoy eating it. I would then just start at the beginning of the book and go through trying all the recipes I fancied.
This way you will still enjoy your cooking, your food will become more varied and you will both find things you like that you may not have even tried before. I would also get your son to join you in the kitchen and help prepare the meals.
I just bought a large nicer dicer from JML (official stand, watch out for fakes) and it makes chopping/dicing fruit/vegetables so much easier. My sons enjoy using it and salads, etc are a lot more manageable. Plus you can 'hid' vegs in meals quite easily as the lumps are quite small.
What I have found is when my sons help with the prep/cooking they enjoy eating the food so much more. The key here is to make a change in the style of cooking and keep stodgy foods in moderation. No need to ban anything really as every once in a while it will be fine. Also have a look at the nutrition facts on food labelling, I have often been shocked by the amount of calories in some innocent looking food.
The other thing to think about is exercise for your son, I know he is in a wheelchair, but is there something he could do to help keep him fit?