There should not be any reason why you can't claim your 17 year old as a dependent. See the rules below. If he incorrectly claimed himself, he will have to amend and you will have to mail in your return.
Qualifying Child
Relationship — the taxpayer’s child or
stepchild (whether by blood or adoption), foster child, sibling or
step-sibling, or a descendant of one of these.
Residence — has the same principal residence as
the taxpayer for more than half the tax year. Exceptions apply, in certain
cases, for children of divorced or separated parents, kidnapped children,
temporary absences, and for children who were born or died during the year.
Age — must be under the age of 19 at the end of
the tax year, or under the age of 24 if a full-time student for at least five
months of the year, or be permanently and totally disabled at any time during
the year.
Support — did not provide more than one-half of
his/her own support for the year
