Why is dieting the only option? Have you considered trying to get him to play more sports? (Admittedly i am making assumptions on his lifestyle)
More exercise is almost always a good idea, however the amount of exercise needed to compensate for a fundamentally unhealthy diet means that the 'more exercise' approach to weight loss is usually doomed to failure.
After seeing my weight steadily rise during my late 30s and through to mid-40s I tried for several years to get my BMI back under 25 by making very minor diet changes and adding more exercise (gym, cycling). It never worked...
Only after hitting nearly 15st (BMI of almost 30 for me) just before my 50th birthday did I decide that I needed to fundamentally change my diet. I used MyFitnessPal to religiously plan and log everything I ate (really
everything - it's really important not to not log the odd snack thinking 'oh it's only one...'). I cut out almost all sugar, cake, bread, processed foods etc. and went all out on fruit & veg, lean priotein, healthy carbs etc. etc. and ran a 500 calorie deficit based on my diet aimed at losing approx 1lb per week, although in reality I lost a little more quickly as I increased exercise as well. It was definitely tough at the start, and I had a lot of cravings for the foods I had cut out, but these gradually reduced as the weight came down.
It took several months to get under BMI of 25 and I still want to lose a few more lbs as I'm still right at the top end of 'normal' - but I feel fitter and healthier than I have done any time in the last decade, and my blood pressure is has moved from being in the 'pre-high blood pressure' section of the chart to smack bang in the middle of 'normal'. It almost feels like it's been a 'life reset' as I really don't need to
try to eat healthy now as it happens automatically and I have very little craving for the unhealthy foods I ate previously - you just have to stick with it long enough for the body to adjust and it becomes the 'new normal'.
/end of motivational/
Driving this behaviour for the population at large is no mean feat. In my opinion the only way would be to massively tax
everything which is too calorie-dense to the point where its unaffordable (i.e. not just focus on one thing like sugary drinks) and use the tax take to subsidise healthy foods.