Firstly, if you donated to your church's general funds and they decided to support a mission that happened to involve your son, and was not influenced by you, then you can take a tax deduction for the donations. But if you donated on condition that the church support your son's mission, or you had influence over the use of your money to support your son, then it is not a legally deductible donation. You can't take a deduction for a donation from which you or your family directly benefit.
Second, the mission is irrelevant to the question of being a dependent.
Third, you can claim a child under age 24 as a dependent if they
1. are a full time student for at least part of 5 months (such as Sept-Dec), and
2. they provided less than half their own support for the year, and
3. they lived in your home more than half the year (more than 183 days). In some cases, you may be allowed to treat the mission trip as a temporary absence and consider the child "officially" lived in your home during that time. It depends on the facts and circumstances.
If your child is 24 or older as of 12/31/25, then you can claim them as a dependent if they have less than $5200 of gross taxable income for the year and you paid more than half their total support.
