I wouldn't be too concerned that he is comfortable with being overweight, teenagers are so critical of themselves I have a hard time believing it's not on his mind. He's probably uncomfortable with having to make changes about his diet when he sees that some people continue to eat crisps without gaining any weight and might not feel open about talking about it. At 14 he's going through a lot of changes, so many that it's hard to take everything in at once. Maybe you can encourage him to take up a sport, or be more active to lose some weight through exercise.
Another thing that might help is trying to make things like biscuits a special treat to have in the house, not just something your family expects to have around. Try to have really healthy meals all week, and make Friday a time to have some of the not as healthy foods.
You sound like a really caring mother and I'm sure your family must appreciate how hard you are trying to ensure that they all have a healthy lifestyle, and leading by example is also great
Good luck!!