I believe you can, as long as you provided more than 1/2 his upkeep for the year and he didn't have more than $4,050 taxable income for the year. From the IRS website:
"In order to claim someone as your dependent, the person must meet certain tests as shown below.
The dependent must be:
Either your qualifying child or qualifying relative
A U.S. citizen, U.S. resident, U.S. national or a resident of Canada or Mexico
Unmarried or if married, not filing a joint return or only filing a joint return to claim a refund of income tax withheld or estimated tax paid.
Additionally, you must meet the dependent taxpayer test. If you can be claimed as a dependent by another person, you can't claim anyone else as a dependent. The requirements for qualifying child and qualifying relative as well as additional information regarding these tests can be found in Publication 501, Exemption, Standard Deduction, and Filing Information."
https://www.irs.gov/forms-pubs/about-publication-501 Here's the IRS interactive guide that may be of use to you.
https://www.irs.gov/help/ita/whom-may-i-claim-as-a-dependent Note that there are massive changes in the IRS taxes for 2018 onwards. For 2018, you can’t claim a personal exemption deduction for yourself, your spouse, or your dependents.