Why can't I claim my son

We can check the criteria to claim your son as a dependent.

But first, have you entered him into the My Info section of your tax return?

1. In My Info
2. Under your name
3. Go through the interview to About Your Household
4. The second question is Do you have any dependents
5. Answer Yes
6. And post the number of dependents (a spouse is not a dependent)
7. Continue the interview and answer all of the questions.
8. Once complete you can [Add Another Dependent] and post your son there.

You can claim your son as a dependent provided he meets the rules below.

• Your son must be a U.S. citizen, a U.S. national, U.S. resident, or a resident of Canada or Mexico.  There are certain exceptions for adopted children.
• You can't claim someone who is claimed as a dependent on another tax return. 
• You can't claim someone who's married and files a joint tax return. 
• They are 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.
• Your child must be under age 19 or, if a full-time student, under age 24. There's no age limit if your child is permanently and totally disabled.
• The child must live with you for more than half the year. Living at school as a full time student does qualify.
• They may have a job, but they cannot provide more than half of their own support.

See the TurboTax article Rules for Claiming Dependents on Taxes