It depends on how much of this simplistic outlook you're willing to inflict upon a loved one. You insist your son is lazy when there might be any number of other reasons why your recommendations aren't working. To me, the complaints sound like there's a lot more going on. If it was simply about feeling a bit overweight, the extra exercise and focus on diet is a simple solution.
I'm not going to advise you look beyond the "lazy" judgement, but I will say that kids today face a world unlike anything we had to grow up with. Their prospects aren't nearly as rosy, they face the judgement of millions of people on social media in addition to their parents and peers, and they see a lot of the lies about society that we failed to pick up on. The pressures they face can't always be dealt with by doing a few more situps, because they're more complicated and reach more deeply.