DISCLAIMER - This answer is not legal advice. No attorney-client relationship is formed. The information provided is for educational purposes only. For specific advice regarding your situation, please consult a qualified attorney in your area.
A disclaimer is not an option, but the probate court can set up a special needs trust for him. This is a specialized trust that would have to be created by a court that allows money that is inherited to be placed in a trust for the benefit of the beneficiary without disqualifying him from receiving state funds. The trust provisions provide that the money is to supplement, not supplant any state funds. The trust would be used for things that the state would not pay for (luxeries, etc.) The trust must also provide that upon his death, the state will be paid off from the trust assets (if available). You would need to find an attorney who is familiar with special needs trusts as it is not a common trust.
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Estate Planning (i.e. wills, trusts, power-of-attorney doc.s, living wills, etc.), Probate and more