The "box-carrimng" of the retinal blood vessels is an indicator of a significant impediment to and/or arrest of blood flow within the retina and in this instance is indicative of the larger process of impaired blood flow in the head in general.
As an ophthalmic surgeon, unfortunately, my experiences with traumatic brain injuries and prognoses for recovery in these cases is limited. The circumstances surrounding this trauma, including a flat EEG and failure to breathe on his own, I think, are likely poor prognostic indicators and speak to what is likely to be a poor outcome. Beyond that, however, I wouldn't have any recommendations that would carry significant clinical weight because of my lack of expertise in this area.
If this is your relative I am truly sorry for this unspeakable event. Please know that my heart and prayers go out to you and this boy's family.
I hope this helps in some small way.
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My opinion is solely informative and does not constitute an informed medical opinion or recommendation. For an informed medical opinion and/or recommendation you must see an eye doctor.