In regards to the situation, both parties are guilty. He has no right to judge you for smoking (unless you're like blowing smoke right in his or his kid's face), and you have no right to judge him or his son for being overweight.
Obesity isn't usually a choice. There are many factors that go into people being overweight. It isn't just inactivity and high calorie diets, although I'm sure there are some cases where that is the sole reason for obesity. Not all societies are set up in a way that is supportive of a healthy lifestyle. The biggest example would be America.
Contrary to popular belief, Americans take less time off, work longer hours, and retire later than almost any other industrialized country's citizens. With less time to be active, people find that as their wallets begin to fatten so are they.
American's also face the challenge of finding normal, healthy sized food portions. Practically any place that serves food sells meals that should only be appropriate for two or more people. Not to mention the availability of healthy restaurants and fast food is scarce. The closest healthy restaurant to me is 50 minutes away, whereas the closest unhealthy one is 5. The cost and availability of healthy foods discourages a healthy diet, and more and more people are finding it more economical and convenient to consume crap.
Obesity also tends to run in the family. There have been studies of identical twins that were raised apart which show weight is strongly influenced by genetics. Not to mention children tend to develop the habits of their parents. When they see the people they want to emulate stuffing their face and then sitting down and watching TV for a few hours, what do you think they're going to start to do? Genes also come into play with health conditions which can cause weight gains through incapability to be active, hormonal imbalance, slowed metabolism, and side effects from medication.
There are a few more factors, but I think my point has been given. I mean what you guys are saying is equivalent to "Let's hate on everyone who causes the slightest bit of annoyance to us no matter how much control they have over the situation." I encourage a healthy lifestyle, but you shouldn't force one upon someone. They just need to realize the benefits and make the change themselves.