hydro and ACC

At 32 weeks gestation my son was found to have Hydrocephalus.  Shortly after birth he had an MRI and was evaluated by a neurosurgeon who believed he was missing structures of the brain such as the Corpus Callosum.  However, two neurologists we took him too disagreed.  They both felt that the hydro was causing his brain to be compressed like a sponge and that these structures were there, but were hard to see.  He was never shunted because the drs felt it was "normal" to my son's makeup and that fluid was being leached through his brain tissue instead of through his ventricles and if they shunted him they were afraid that his brain would collapse.

Today my son is nine and up to this point has been developmentally normal.  He reached all of his milestones at the right time(walked at ten months, etc.)  Academically he has been doing very well. However, this year some issues have arisen.  For example he does well in class work, but has trouble taking tests.  He also has started having trouble with reading comprehension. When his teacher stated that she feels he is working to his best ability and has fallen below average we decided to get him tested.

He was evaluated by a dr who specializes in childhood learning disabilities.  She stated he has two major issues.  One is that he is slow to process information(hence poorly on tests) and two that his visual/cognitive skills are very poor (hence comprehension problems).  She is amazed that as bad as his testing came back that he is able to perform so well academically(70%).  She believes both of these things are directly related to his hydro.  But she also insinuated that we need to get him back to a neurologist as this could get worse as more complex thinking is required in higher grade levels.  

My questions are these.  If he did have Agenesis of the Corpus Callosum wouldn't there be more signs before now?  Also are these learning problems common in children who have hydro? I have gotten him an appointment with a neurologist, but it isn't for another month.