One person is a dependent of another person or not. It is not a choice. Your first step is to determine if the 18 year old son is a dependent of the father. There are 5 dependency tests: 1) Relationship 2) Age 3) Residency 4) Support and 5) Joint Return. All 5 tests must be met to claim someone as a dependent. See IRS Publication 501 for an in-depth discussion on dependents and exceptions to the various tests.

For example: Let's say the son provided $10,800 of his own support and the father provided $11,000 of the son's support. The son met all the other dependency tests. The son is a dependent of the father. But, if the son provide $10,800 of his own support and the father provided $10,000 of the son's support. Even if the son meets all the other dependency tests, he is not a dependent of the father and cannot be claimed on his return.