My son was only a halftime student last year, so i cannot claim his an a dependent. but i paid for his tuition. how do i claim this expense?

Unless your son is a dependent on your tax return you cannot get the education credit.  

https://ttlc.intuit.com/questions/3383321-why-didn-t-i-get-a-credit-or-deduction-for-education-expenses

However, if your son files a return he can claim the education credit since he is not a dependent.

In order to claim education expenses two criteria must be met.

 - The student must qualify as your dependent

 - You must actually claim the student as your dependent.

If you don't meet both criteria, then you can't claim any expenses related to the student or their education. The student will claim it all, regardless of who actually paid it.

For other's reading this:
Student defined.

To qualify as a student, your child must be, during some part of each of any 5 calendar months of the year:

1.A full-time student at a school that has a regular teaching staff, course of study, and a regularly enrolled student body at the school; or

2.A student taking a full-time, on-farm training course given by a school described in (1), or by a state, county, or local government agency.The 5 calendar months don’t have to be consecutive.Full-time student.

A full-time student is a student who is enrolled for the number of hours or courses the school considers to be full-time attendance.School def