I love it when someone asks me a question with a clear answer! Here, the answer is probably not. Your son receives Social Security Disability Insurance (SSDI), which is a benefit that he receives because he was able to work and pay into the Social Security system. This is not a needs-based program like Supplemental Security Income (SSI), which people receive when they are permanently and severely disabled and qualify for this assistance because their resources are limited. Medicare is an age-based health insurance program, so it’s not needs based either.
Special Needs Trusts are designed to allow parents (and others) to create a trust for the benefit of someone on SSI that can be used to supplement this government benefit without disqualifying someone from the program (and from Medicaid programs, which offer health insurance to those who qualify for SSI). If your son is not going to need SSI in the future, and will continue to receive SSDI and Medicare, then a Special Needs Trust won’t be necessary to protect his ability to continue to receive those benefits. That being said, of course, it still sounds like you need to leave him assets in a trust, with a Trustee who can manage resources for his benefit. This trust, however, does not need to meet the stringent requirements of a Special Needs Trust.
To read more about Special Needs Trusts and the differences between SSDI and SSI, take a look at Special Needs Trusts (Nolo, 5th edition).