Star -
You can always amend a previous return if you believe you made a mistake that will significantly affect your tax obligation. You should review Publication 501 for the tax year you want to amend to make sure that you can claim that person as a dependent. It sounds like you can but you should double check if you're going to amend a return. You then submit form 1040X with the changes. You can find form 1040X and it's instructions at the IRS website
www.irs.gov
One thing to be aware of when you amend a previous year's return. It starts the clock ticking again for the IRS to question or audit the return for up to 3 years from the time you amend the return.
If you do amend a return for this reason, I suggest you add a statement at the end explaining that you are only amending the return because you overlooked claiming your son as a dependent.
Busy -
If your son is over 18 and not a student (as defined by the IRS) then you cannot claim him as a dependent child. He may however meet the rules for dependent relative if his earned income for the year is less than $3650 and he meets the other tests of dependency. See publication 501 if you're not sure or look at the instructions for Form 1040.
http://www.irs.gov/publications/p501/ar02.html#en_US_2010_publink1000220868
Everyone with an adult dependent child who is a student should be aware of the IRS rules regarding grants, loans or scholarships. Depending on your particular situation, grants, loans and scholarships may make it difficult for you to show you provided more than 50% of your adult student's support or may result in your loss of the child as a dependent because their earned income exceeds $3,650.
That's because sometimes these sources of income are treated as unearned income to the student and other times as earned income to the student. For example, loans count differently according to who took the loan out and what it was used for. If a loan was taken out by the child then that unearned income may affect your ability to show that you provided half the child's support if the loans paid for housing and food expenses.
When in doubt check the IRS publications about the specific subject areas at
www.irs.gov. The info in the 1040 instructions is often not complete enough to answer more detailed questions about dependents and exemptions.
Patricia