The 10% number is not going to be accurate.
The biggest factor will be what exactly was damaged & how badly damaged it is. (I'm talking cellular level damage, which individual 'circuits' are disrupted)
The brain is a network of cells, it's not capable of repairing any damage that kills cells, but can re-route information around small areas of damage so long as there's enough healthy tissue. This is a long process of re-learning basic skills.
Getting to a trauma centre quickly probably saved his life. They have the skills & resources to treat brain inflammation which is a major killer. Being treated there gave him a better chance of recovery.
His age will give him a better chance to re-train his brain & re-route information around any permanent damage.
Opening his eyes & reacting to stimuli are very good signs. Read up on the Glasgow Coma Scale so you can have an idea of how badly he's affected once he's taken off sedation & see any improvements.
With brain injuries you're looking at so many variables that trying to pull meaningful statistics for a case will be very difficult, you'd need access to case files for multiple patients with the same regions damaged to a similar extent.
You may find something useful in a specialist medical journal like 'Brain Injury' or 'The Journal of Head Trauma Rehabilitation' (the first 2 that a quick google threw up), a keyword search in
PUBMED would also be a good place to start
There are parents of kids with rare conditions who know it's details better than their consultant, and their knowledge is up there with the best in the world, it's down to how much they read up on the topic.