Is my disabled 24 year old son who works 9-12 hours per week still considered disabled for tax purposes?

If he is working then he is not disabled for tax purposes.

The IRS considers you to be disabled if you can't engage in any substantial gainful activity because of a physical or mental condition. You must have a physician certify that the condition can be expected to last continuously for 12 months or more, or can be expected to lead to death. Here is an excerpt from IRS publication 524: