My child is 19 and was in high school last year but took college courses in high school. TT keeps assuming he's in college. How do i input this info?

In the personal info section , you need to answer that he was a full time student.  This helps qualify him as your dependent, because he's over 18. 

Answer no to being a student on the other education questions, unless you are trying to claim the tuition credit. But, it really doesn't matter if he is identified as a college student, unless you are trying to claim a tuition credit. In that case, TT will ask the right qualifying questions. 

The ability of the parent of a high school student to claim the education credit is limited.  If these courses are provided by an eligible college, this cost qualifies for the Lifetime Learning Credit (LLC). The school must have policy of granting college credit for that course, already taken, if the student ever enrolls there. In other words, it is a requirement that the course be a college credit course, even if the student isn't currently a college student. It’s not that the student is post-secondary, it’s that the course is post secondary. Books are not a qualifying expense, unless included in the course fee. You must have paid tuition to the college and not the high school in order to claim the expenses on your return.

https://www.taxact.com/support/22177/2023/education-credits-high-school-student-taking-college-classes?hideLayout=False

High school students are not usually eligible for the more generous American Opportunity Credit, unless they are officially enrolled as a degree candidate.