As long as he indicated that he was being claimed as a dependent when he filed you should be fine.
Please view the information below if you want to know if you qualify to claim him.
Qualifying Child Rules:
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Are they related to you? The child can be your son, daughter, stepchild, eligible foster child, brother, sister, half brother, half sister, stepbrother, stepsister, adopted child or an offspring of any of them.
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Do they meet the age requirement? Your child must be under age 19 or, if a full-time student, under age 24. There is no age limit if your child is permanently and totally disabled.
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Do they live with you? Your child must live with you for more than half the year, but several exceptions apply.
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Do you financially support them? Your child may have a job, but that job cannot provide more than half of her support.
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Are you the only person claiming them? This requirement commonly applies to children of divorced parents. Here you must use the “tiebreaker rules,” which are found in IRS Publication 501. These rules establish income, parentage and residency requirements for claiming a child.
If your child was age 24 on 12/31/2018 then the Qualifying Relative rules apply.
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Do they live with you? Your relative must live at your residence all year or be on the list of “relatives who do not live with you” in Publication 501. About 30 types of relatives are on this list.
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Do they make less than $4,050 in 2017? Your relative cannot have a gross income of more than $4,050 in 2017 and be claimed by you as a dependent.
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Do you financially support them? You must provide more than half of your relative’s total support each year.
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Are you the only person claiming them? This means you can’t claim the same person twice, once as a qualifying relative and again as a qualifying child. It also means you can’t claim a relative—say a cousin—if someone else, such as his parents, also claim him.
