First of all, concerning the W-4 form, you can fill out that form and claim one or two or even three dependents, regardless of whether or not you actually have that many to claim on your tax return at the end of the year. The W-4 form is basically just a worksheet used to determine how much taxes you think you need to have withheld. People who have higher deductions than normal may claim more exemptions, knowing that at the end of the year they will have enough deductions to take, so they claim more exemptions to have less tax withheld from their checks.
However, concerning your actual right to claim your son as a dependent:
To claim your son as a dependent, he must be uder age 19 - or under age 24 if a full time student.
He must live with you for more than half of the year and he must not have provided more than half of his own support.
If your son meets the rules to be a qualifying child of more than one person (such as an ex spouse), then you must be the person entitled to claim him.
If he meets these guidelines, you may claim him as a dependent on your tax return.
I am giving you a link to Publication 501. Refer to the section entitled "Exemptions for Dependents"
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