If I still claim my 18 year old as a dependent at the end of the year , should he claim zero or one on his W4? He is a full time student with a part time job.

Yes. 19 is the cut off age and that is raised to 24 for full time students. Continue to claim him on your w-4.

A child of a taxpayer can still be a “Qualifying Child” (QC) dependent, regardless of his/her income, if:

1. He is under age 19, or under 24 if a full time student for at least 5 months of the year, or is totally & permanently disabled

2. He did not provide more than 1/2 his own support. Scholarships are considered third party support and not as support provided by the student.

3. He lived with the parent (including temporary absences such as away at school) for more than half the year

So, it doesn't matter how much he earned. What matters is how much he spent on support. Money he put into savings does not count as support he spent on him self.

Furthermore, there is a rule that says IF somebody else CAN claim him as a dependent, he is not allowed to claim his own exemption. If he has sufficient income (usually more than $6350), he can & should still file taxes; he just doesn’t get his own $4050 exemption (deduction). In TurboTax, he indicates that somebody else can claim him as a dependent, at the personal information section.