I think a parent has almost full control of a child's diet until they are about ten. Birthday parties and other celebrations aside, children tend to eat what is in the fridge/cupboards and snack on foods readily available. If only crisps, sweets and processed/microwaveable fast food is at hand, what does a child do?
I don't want to sound holier than thou but the importance of diet has been thrust in our faces for years. We all know what a decent diet comprises of and the importance of fruit and vegetables. As teenagers, out and about on their own, with friends and pocket money on them I suspect McDonalds and other fast food outlets made a fortune from their visits. I despaired but they were active and at home they had to eat what was served (which was always reasonably healthy) or go hungry. I also know one of my sons and his friends ate seconds most days or all the left-over pizza at school for as long as he could get away with it. One of the dinner ladies, a local Mum, told me, long after the event.
Kids do like junk food. They'll choose chocolate over an apple any day given the choice. (Don't we grandparents know it!!) A parent has to start them on the right path with the 'good stuff' though. I think it's a duty. I don't know his parents' circumstances and I feel sorry for the boy and them, but I get the feeling his blindness was preventable, if a lack of nutrients caused it.