In general, support includes amounts spent to provide food, lodging, clothing, education, medical, dental care, health insurance, recreation, transportation, etc. Lodging expenses would be the fair market value of the lodging.
The question to answer is: Of the expenses you provide to him, is that greater than 50% of his total expenses. If he is providing his own lodging, education expenses, food, clothing, etc, and that total is the majority of his expenses, then he is correct to claim his own exemption.
You can use the IRS worksheet below to determine the support level you provide for your son.
IRS support worksheet
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