As runner pat already said, but it bears re-emphasizing, the effects of an injury like this depend very much on which particular parts of the brain get damaged (and, of course, how much of it). Different parts of the brain do Different things (and, what is more, they are arranged slightly differently in Different people) so without knowing precisely which parts of this poor boy’s brain it is impossible to say exactly how he will be affected, and even his own doctors, who do know precisely where the spear went, will not be able to say exactly what the effects will be yet.
He will certainly be mentally impaired, but in what sort of ways it is impossible for anyone to say yet. Some of the effects of brain injuries can be downright weird. Looking at the X-ray, I would guess that it is unlikely that he will be able to live independently, like Phineas Gage did, but that is only a guess. Gage’s injury was pretty much confined to his frontal lobes, sort of like a getting a lobotomy, whereas it looks as though in this case it goes through the frontal lobes, through the middle of the brain and out through the occipital or parietal cortex (which means his vision and/or his sense of spatial relations, and his personallity, will probably be affected, amongst other things, no doubt). Fortunately, much of the middle part of the brain is just fluid filled cavities anyway, and much of the rest is white matter (cables rather than processors) so it does not matter quite so much as the cortex (outer surface) where most of the processing goes on. But some cortex has clearly been destroyed, and cables are important too. The mid-brain and brain stem, which are responsible for basic functions that keep you alive, seem to have been missed altogether, which is good.
As runner pat already said, but it bears re-emphasizing, the effects of an injury like this depend very much on which particular parts of the brain get damaged (and, of course, how much of it). Differnt parts of teh brain do differnt things (and, what is more, they are arranged slightly differntly in differnt people)