"Sturdy" grandson.

Recently I've noticed my 13 year old grandson has gained a lot of weight. He's not in the least interested in physical pursuits, preferring to spend his time glued to his iPad. My son is happy with this, as he says at least he knows where he is, and what he's doing.
Now, I would answer this thread and tell myself it was none of my business, it's up to his parents, but I just wondered if anyone had any suggestions.
He is a very sociable child ( teenager?), and certainly not lacking in social skills. I just feel it would be easier for his parents ( both slender, as are his younger brother and sisters) to address the issue now.