Can I claim my 18 year old if he worked in another state for the summer?

Since he was only temporarily away from the home you would indicate in the section for adding a dependent that he lived with you for the entire year.  A child under the age of 19 can be claimed as a dependent regardless of their income as long as they meet all the Qualifying Child rules.

If he files a tax return make sure that he indicates on his tax 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.