I'm the family cruise editor for CruiseMates.com. Holland America Line's youth program is only offered to children ages 5 to 17 years. (Many other programs start at age 3.) If your son is over 5 and can participate, HAL offers the usual array of on board activities for children but the hours of operation for the youth program are not quite as long as some other cruise lines which carry very large numbers of children (such Carnival, Disney or Royal Caribbean).
For more details on HAL's youth program, read the article I recently wrote for the family web site titled: "Kids' facilities and activities by cruise line."