"By him marking the box on the personal info sheet of his return that someone can and will claim him as a dependent on their tax return, his return significantly swings from a nice refund to having to pay a little tax."
What you are seeing may be the result of an error. If someone CAN claim him (and you can) you cannot answer the "someone can question" with a no. You can answer the "someone will question" with a no.
While technically there is a provision that allows your student-dependent to claim a federal tuition credit, from a practical matter it seldom works out. A full time student, under age 24, is only eligible for the refundable portion (usually $1000) of the American Opportunity Credit (AOTC) if he/she supports himself by working. He cannot be supporting himself on student loans & grants and 529 plans and parental support. It is usually best if the parent claims that credit, if eligible.
If the student actually has a tax liability, there is a provision to allow him to claim a non-refundable tuition credit. But then the parent must forgo claiming the student as a dependent, and the $500 other dependent credit. The student must still indicate that he can be claimed as a dependent, on his return. This is worth up to $2500 (AOTC shifts to all non refundable)
