Does the Advanced Individual Training school count toward being in school, per tax purposes? If I cannot claim him, will I be able to claim him as a dependent once he starts school in January?
No, you cannot claim him as dependent in 2017, unless he was still in high school in the first part of the year.
Army training, including Advanced Individual Training, does not count as being a student for tax purposes. Ye, you should be able to claim him as a dependent once he starts school in January.
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 or military training) for more than half the year