It's not the number of months, in a year, that define full time or half time student. It's the number of hours in a week that the student attends class. Each school defines full time for their curriculum. "Full-time student" typically means a student who is scheduled to attend a minimum of twenty or more clock hours or twelve or more credit hours per week.
Even though the 1098-T is not checked for half time, the TT interview gives you a chance (after entering the 1098-T) to declare that you student is half time or more based on your understanding of his status.
