1.Joubert syndrome is similar to Dandy Walker in that there is vermal agenesis, but it also includes a characteristic face that docs can recognize, eye movement problems, and difficulty breathing due to problems with the respiratory centers of the brain. These do not appear to be associated with Dandy Walker.
2.It is uncommon but possible to get seizures from hydrocephalus.More common when there is a shunt.
3.As for the polycystic kidneys, there is a big difference between the polycystic kidneys that are associated with syndromes such as Dandy Walker and the more common Adult POlycystic Kidney disease that is seen in the general population. The former is just associated with the congenital syndrome whereas the latter is something different in that it is seen in patients who are apparently normal and is a disease all its own. Adult Polycystic kidney disease is hereditary in what's called an "autosomal dominant" fashion, which means that if you yourself have the disease, your child has a 50% chance of getting it. But if you or your spouse do NOT have the disease, then your child theoretically is safe from adult polycystic kidney disease (but not necessarily polycystic kidneys in association with congenital disease).
Hope that helps, and happy birthday to your son.