The presence of atrophy of the left parietal and occipital areas indicates that he probably suffered a disrupted blood supply to those areas, most likely when he had a subarachnoid haemorrhage.
Children at this age are blessed with brain plasticity, meaning that other neighbouring areas of the brain will try and compensate to an extent.
The problems that could arise and you should be able to detect them:
1) Seizures or convulsions. Look out for any jerks that occur after he wakens from sleep.
2) Weakness or stiffness of the right arm and leg. If a child preferably uses one hand or leg before 2 years of age, you should look for weakness of the opposite limbs. Do not massage his limbs.
3) Delay in walking and/or speech
4) His developmental milestones will be periodically assessed by his Neurologist and appropriate intervention done.