There is more than one possible answer.

a) It is a son that is affected. So it could be an X-linked disease. If so, then genotypes are

1. N/- (where N normal, – is Y chromosome with no gene).
2. Nn.
3 and 4 NN or Nn.
5. n/- The son has been affected as he has inherited the disease from his mother’s X chromosome.

If it is not X-linked then

1 and 2. will both be Nn.
3 and 4. could be NN or Nn.
5. nn.

b) X-linked disease:
The probability that the next child will be affected is 50% for a son, depending on whether he inherits the normal or affected chromosome from his mother and 0 if it’s a daughter as she will always inherit the normal X from her father. Daughters have a 50% chance of being carriers.

Autosomal disease:
In this case, there is a 25% chance that the next offspring will be affected (there is a 50% chance of inheriting n from each parent, 0.5 x 0.5 = 0.25).

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