For your boyfriend to claim a person as a dependent – that person must be either his qualifying child, or qualifying relative.
Here are Tests To Be a Qualifying Child
1.The child must be your son, daughter, stepchild, foster child, brother, sister, half brother, half sister, stepbrother, stepsister, or a descendant of any of them.
2.The child must be (a) under age 19 at the end of the year and younger than you (or your spouse, if filing jointly), (b) under age 24 at the end of the year, a full-time student, and younger than you (or your spouse, if filing jointly), or (c) any age if permanently and totally disabled.
3.The child must have lived with you for more than half of the year.
4.The child must not have provided more than half of his or her own support for the year.
5.The child is not filing a joint return for the year (unless that joint return is filed only as a claim for refund).
Because your boyfriend is not related to your son – your son may not be his qualifying child.
Here are Tests To Be a Qualifying Relative:
1.The person cannot be your qualifying child or the qualifying child of any other taxpayer.
2.The person either (a) must be related to you in one of the ways, or (b) must live with you all year as a member of your household2 (and your relationship must not violate local law).
3.The person's gross income for the year must be less than $3,650.
5.You must provide more than half of the person's total support for the year.
#1 would be satisfied if your son is not the qualifying child of any other taxpayer. Assuming only you live with your son and you do not work and actually are a dependent yourself – that test is satisfied.
#2 and #3 seem as satisfied – please verify.
$4 – is critical – it is satisfied only if your boyfriend provided more than half of your son’s total support for the year.
So as long as all these tests met – your boyfriend may claim your son as a Qualifying Relative (but not as a Qualifying Child).