Can he still be your dependent? That will depend. If he earns less than the "exemption amount" (in 2018 this amount was $4150, and I believe it is $4200 next year), the answer is yes as long as you provide at least half of the support for him. He would at least qualify as your qualifying relative.
But the real question is whether he can be considered your qualifying child, which carries a much greater benefit to claim your child. It's not automatic. While a totally and permanently disabled child may be claimed "perpetually" as a qualifying child, one of the IRS' determinations is if he can participate in any substantially gainful activity. Facts and circumstances will dictate, but please see the following IRS link: Qualifying Child Rules, and note the following definition:
Permanently and totally disabled. Your child is permanently and totally disabled if both of the following apply: The child cannot engage in any substantial gainful activity because of a physical or mental condition and a doctor determines the condition has lasted or can be expected to last at least a year or lead to death.
If the child's "part time job" constitutes substantial gainful activity, then this may be the last year you may claim him as a qualifying child. (He may still qualify as your qualifying relative as I mentioned above even if this is the case).
Your other question is how to report his SSDI? You don't report it on your return. If your son files a tax return, the amount is included with his income, but it is not income that you claim on your return (and is likley not taxable at all).
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