Under Trump will anything happen to my brothers Social Security Disability? He is 42 and draws it for mental illness.

If Trump somehow can eliminate taxes on social security, the fund will have to reduce benefits sooner than projected. The cut will only benefit 40% of recipients that pay taxes, and that lack of extra funds comming into the system will bring it closer to insolvency, sooner rather than later. So unless your brother is one the 40% that will immediately benefit from the tax cut, he'll get to look forward to lower payments sometime in the nearer future.