Assuming there's a plan whereby the S-corp routinely reimburses health insurance premiums for employees (possibly for certain classes of employees, but "the shareholder's adult kid" doesn't constitute a class), then yes, the premium reimbursement would be treated as SE health, includable in Box 1 but not 3 or 5, and deductible at the 1040 level under the deemed shareholder rules.

If no plan, it's still deductible by the employer, goes in Boxes 1, 3, and 5, and arguably is not deductible at the 1040 level due to lack of plan.