Not as a dependant. E-2 derivatives are the spouse and unmarried children under 21. An adult son would need his own basis, and E-2 employee status for him would require a role that is executive, supervisory or essential to the enterprise.
Adult children need their own route
Hypothetical example: a Rothesay couple's 26-year-old son manages a hardware store and would be the natural person to run their U.S. storage facility. He cannot accompany them as an E-2 dependant because he is over 21.
The E-2 category does allow a treaty enterprise to employ nationals of the treaty country in executive, supervisory or essential-skills roles, so if he is a Canadian citizen and the facility genuinely needs a manager at that level, an E-2 employee application in his own right could be examined. That application would be assessed on the enterprise's need and his qualifications, not on the family relationship, and a facility that is itself close to marginal may struggle to justify the role. Alternatively, if he contributes qualifying capital and will develop and direct the business, a principal-investor application could be examined if he owns at least 50 percent or possesses genuine operational control; his parents may remain co-owners if the treaty-nationality and control structure is accurately documented.
The family should choose one structure and evidence it.