"I've linked it with the W2 he received from S corp."

That link makes no sense to me, either. A W2 by definition means you are not Self-employed, and you stated this amount is not listed on the W2 because the corporation was not involved in this activity.

"This is then generating, I believe, the SE insurance deduction on line 16, Schedule 1"

The link you seek would be from a 1065 K1.

I found this great resource for you:

https://politoeppich.com/articles-of-interest/partnership-and-s-corporation-health-insurance-premium...