Not as a dependant; he is over 21. He could be employed by the enterprise as an E-2 employee only if he qualifies as an executive, supervisor or essential-skills worker and holds Canadian citizenship, which requires its own visa application.
An adult child is an employee or nothing
Hypothetical example: a Frontenac Islands couple's son, 24, has run the cottage-rental side of the family business for three years and expects to manage the campground's cabins. He is not a derivative because he is over 21, so his mother's and father's E-2 status gives him nothing. The enterprise could sponsor him for an E-2 employee visa if he is a Canadian citizen, the enterprise's ownership is Canadian, and his role is either supervisory or executive or one requiring skills essential to the business.
A cabin manager supervising seasonal staff might meet the supervisory test; a general seasonal hand would not. The essential-skills route is harder for hospitality work that a U.S. worker could learn.
If he does not qualify, he may visit without working. The couple's E-2 spouse, whichever is not the principal, is employment-authorized incident to status, so the household's working capacity is two adults, not three, unless the son qualifies on his own. Hypothetical example: A Frontenac Islands couple buy a document-storage company and plan to bring a twenty-two-year-old daughter who handles their Canadian bookkeeping.
She is too old to be a derivative. The first review should decide whether she has her own viable category, a properly qualifying employee role, or a plan that does not involve work. Her passport and birth certificate settle age eligibility; a title created for the application does not establish an executive, supervisory, or essential-skills position.