The tax programs will walk you through most of this now a days but you are going to be able to decrease your taxable income by about 5k by having him as a dependent, 3 children, plus 2 under 17 for the child credit, plus 3 on the income tax credit. plus he technically was a dependent from what you are saying. So yes, his few hundred dollar refund will end up being like $100 or less instead but it will make a big difference to you, especially if your income is 1099 or additional income. So assuming you 2 aren't in a huge fight or something then you should definitely claim him.