There are a lot of fishy details here. First, I would be willing to bet those are not really his feet. Not only do they simply look very unlike a child's feet, there are some obvious deformities and issues that are not consistent with his young age,and apparent good condition with the rest of his unscathed body. A human living in the wild would be hindered by these claw-like toenails. I would even say they would not even be able to grow that long in the first place; walking around in the wild barefoot, they would get caught on branches, plants, etc. and break off naturally as they grew too long, or, because of this inconvenience, the boy would have learned to bite or tear them off.
On the tops of the feet are what look like age spots. Freckles maybe, but they look very much like age spots. The person who has these feet most likely has rheumatoid arthritis, and that is evidenced in the nodules in the joints of the lesser toes on the left foot. Also, a probable condition called 'claw toe', (nothing to do with the nails--rather, the curling, flexed shape of the toes themselves) in which the lesser toes become compromised due to a high arch that does not flatten properly when walking. Looking at the right arch that is visible, it does seem high. My guess is that these feet belong to an older person, and the dastardly condition of the toenails is due to an issue of immobilty and inability to care for one's self. The feet are the first thing people usually stop taking care of when they are sick, injured, and/or disabled. Sometimes it is physically impossible.
The footprints of aboriginals clearly show that walking barefoot in grasses and on natural landscapes help the foot maintain a much more healthy shape over time than our shoe-wearing, pavement pounding feet. So, I don't think the 'wild' factor itself could account for the deformities, especially with a person this young. Also, there is evidence of what may be surgical scaring/sutures directly under the toes of the left foot. Never trust a wolf in a white coat wielding a scalpel.
It also looks like these feet may have some minor bunions, and there's something going on with the the lateral area of the left foot too.
Of course, if these are the poor child's feet, these medical issues may be severe enough to account for his 'walking with his legs half-bent.' I'm not familiar with juvenile arthritis specifically, but I believe it is essentially the same disease in children as in adults. If he has a severe case of some type of rheumatic disorder, believe me, his untreated pain could also perhaps account for his snarling and bizarre behaviour.
The article states the boy looks about 10 years old, but inexplicably says the think he may be 'much older.' It says all they had time for before the escape was a shower and blood work, and other tests. I cannot imagine any type of test that would indicate an older age (or any age, for that matter.)