Unmarried children under twenty-one may accompany the investor as derivatives, and age-protection rules can preserve eligibility for a child who turns twenty-one while the petition is pending, depending on filing and visa-availability dates. He should not be counted among the ten qualifying employees, and his own employment in the United States must wait for his own status.
Calendar the child's age against the filing dates
A nineteen-year-old gives the family some room, but the timeline should be mapped. Age is measured under the applicable protection rules by reference to the petition's filing and approval dates and the availability of a visa number, and the details depend on the family's category and country of chargeability. File the I-526 as early as the capital and evidence allow, and keep the son's status as an unmarried child intact until the family's residence is granted.
Do not put him on the US shop's payroll as one of the ten workers; the investor's spouse and children are excluded from the count and his employment before he holds status of his own would be unauthorized. If he wants to keep working in the family business in Canada, that is unaffected. Once the family holds conditional residence he may work freely in the United States.
If the timeline is tight, discuss whether a student route or a separate employment route might serve him better than waiting as a derivative.