If my child was "at least a half-time student" for one semester in 2016, can he qualify as a dependant

[Hal_Al]

No.

Your child over age 18, must be a FULL TIME student for parts of 5 calendar months to be considered a Qualifying Child Dependent*. 

The half time student rule is about the course work qualifying for an education credit. It is not about  dependency qualification.

*There are two types of dependents, "Qualifying Children"(QC) and standard ("Qualifying Relative" in IRS parlance even though they don't have to actually be related). There is no income limit for a QC but there is an age/student status test. Your adult child may still qualify as a standard dependent if his income for the year was less than  $4050.