You can still claim your son as a dependent under the Qualifying Child rules if he meets all the requirements. His income is not relevant under the rules.
If his income is from wages reported on a W-2 he can file a return to get a refund of the taxes withheld. Income of $6,300 or more reported on a W-2, he must file a tax return. If his income is $400 or more from self-employment and reported on a Form 1099-MISC or cash then he must file a tax return to report his self-employment income and expenses. Assuming that you will be claiming him as a dependent, then when your son files his tax return we would indicate on the return that he can be claimed as a dependent on someone else's tax return.
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, (b) under age 24 at the end of the year and a full-time student or (c) any age and permanently and totally disabled.
3. The child must have lived with you for more than half of the year. Temporary absences while away at college are considered living with you.
4. The child must not have provided more than half of his or her own support for the year.
5. If the child meets the rules to be a qualifying child of more than one person, you must be the person entitled to claim the child as a qualifying child.
6. The child must be a U.S. citizen or U.S., Canada or Mexico resident for some portion of the year.
7. The child must be younger than you unless disabled.
