This depends on whether or not your child meets the dependency requirements as either a qualifying child, or qualifying relative. Theoretically, you could claim your child forever, as long as he or she meets the guidelines. See the following:
Rules for all dependents include:
All dependents are either Qualifying Children or Qualifying Relatives.
Will he be living with you still?
What is his age and how much will he make?
If he is under 19, then he will be your dependent "qualifying child" as long as he does not pay more than 1/2 the cost of supporting himself.
If he is between 19 and 23 (not 24 at the end of the tax year) then he can still be a dependent "qualifying child, as long as the school he is attending is an elementary, junion or senior high school, college, university, or technical, trade, or mechanical school--and he attends for at least 5 months of the year as a full time student. HOWEVER--- an on-the-job training course does not count as a school.
From your description, it sounds as if he is not attending a vocational school, but an on the job training program. Therefore, if this is the case, and he is making 6-10 grand per year and over 19, then you cannot claim him as a dependent.
Please let me know if you need any additional clarification.