musukosan
むすこさん
(someone else's) son
Note
[musuko san]…refers to somebody else's son, while "musuko" refers to one's own son.
Example
A：むすこさん、むすめさんは おいくつですか。
B：むすこは ろくさい、むすめは にさいです。
A: How old are your son and daughter?
B: My son is six and my daughter is two.