Will claiming my boyfriend and his son for the first time throw a red flag on my return? He didn't work and it's his year to claim him, so instead, I am claiming both.

If the son is not also your biological son, you can only claim him as a qualifying relative dependent not a qualifying child dependent. Both of them had to live with you the entire year. Here are the requirements that each of them must meet to be claimed by you as qualifying relative dependents:

1. The person cannot be your qualifying child or the qualifying child of any other taxpayer. A child is not the qualifying child of any other taxpayer if the child's parent (or any other person for whom the child is defined as a qualifying child) is not required to file an income tax return or files an income tax return only to get a refund on income tax withheld.

2. The person either (a) must be related to you or (b) must live with you all year as a member of your household. 

3. The person's gross  taxable income for the year must be less than $4,150 in 2018.

4. You must provide more than half of the person's total support for the year.

5. The person must be a U.S. citizen, U.S. resident alien, U.S. national, or a resident of Canada or Mexico.