Here's another perspective. I, too, have a 16 yr old who plays A LOT of computer. My husband and I are aware of the worst case scenario i.e. dehydration, not enough calories and severe lack of sleep in the short run and having nothing else in his life in the long run. (you heard the possibilities from others...)
Having said that, we see some positives in the gaming (call it denial, but we call it positive thinking,..) Our son plays World of Warcraft and, like many of these games it is DESIGNED to keep them playing and playing and playing... However, it also requires him to plan ahead, cooperate with others, memorize a LOT of facts and know how to apply a LOT of techniques, follow a map, manage finances (of a sort), and it is a great source of accomplishment and pride for him.
For a kid with ADHD, WOW is actually teaching him (or allowing him to use) some valuable skills. When school, and the world in general, is NOT built for him, WOW gives him a venue for creating his own world that honors his gifts.
OK, I know this is a computer game, and it is not a substitute for friends, family and physical activity, but I want to express the other side so you can not despair!
And for the food...my son has ALWAYS eaten weirdly (no breakfast, often no lunch and then an after school meal, a bit of a meal with the family and another meal before bed. Now it's summer and he just grazes from time to time. We make sure there are easy and reasonably healthy things around for him to grab (healthy cereal, frozen foods, fruit, nuts...) Hearing stories of weird eaters who make it is affirming. One more? My sister ate ONLY vienna sausages and Tang for months while she was a little kid...and now she is a SUPER healthy VEGETARIAN. Go figure.
Love you grandson a lot, pay attention to his game (it IS what is important to him) NOTE: you don't have to understand it to listen to him, just nod and smile and ask him how it's going from time to time. And love him some more. Kids do weird things, they always have (I'm sure we did). You'll make it.