Of course some people will use this as an excuse to do nothing to change their lifestyle and expect everyone else to adjust for them, but those people would find some other reason anyway so there is no point worrying about them.
And the best be to counter this, if it is actually possible, is to point out that while he might be genetically predisposed to a slow metabolism that burns less that means he needs less calories not that he has an excuse to be fat. It means he needs to either burn more through exercise or eat less. Genetics is the reason he needs to do that to stay in shape not an excuse not not bother about his health. Not that he'll listen. If he is someone you like enough to make the effort (or you want to win the intellectual discussion) try use other conditions as a comparison to see if you can get through that way. Anaemia, a genetic condition, is the reason some people have trouble with wounds not clotting and healing properly, it isn't something they use as an excuse to bleed everywhere, get infected wounds, and so forth.
I've recently turned my health around (down from more than 17 stone 18-to-24 months ago to just under 10 stone now, and running 5K at least twice a week (sometimes 10+) where I couldn't run 2K in one go in January). It is slightly irritating that some people can eat a shed load more than I can while not putting on weight, but that didn't mean it was impossible for me to get into a reasonable shape and maintain it. The reason I didn't before was just that I didn't care enough to.
Of course some people will use this as an excuse to do nothing to change their lifestyle and expect everyone else to adjust for them, but those people would find some other reason anyway so there is no point worrying about them.
And the best be to counter this, if it is actually possible, is to point out that while he might be genetically predisposed to a slow metabolism that burns less that means he needs less calories not that he has an excuse to be fat. It means he needs to either burn more through exercise or eat less. Genetics is the reason he needs to do that to stay in shape not an excuse not not bother about his health. Not that he'll listen. If he is someone you like enough to make the effort (or you want to win the intellectual discussion) try use other conditions as a comparison to see if you can get through that way. Anaemia, a genetic condition, is the reason some people have trouble with wounds not clotting and healing properly, it isn't something they use as an excuse to bleed everywhere, get infected wounds, and so forth.
I've recently turned my health around (down from more than 17 stone 18-to-24 months ago to just under 10 stone now, and running 5K at least twice a week (sometimes 10+) where I couldn't run 2K in one go in January). It is slightly irritating that some people can eat a shed load more than I can while not putting on weight, but that didn't mean it was impossible for me to get into a reasonable shape and maintain it. The reason I didn't before was just that I didn't care enough to.