A chapter 13 bankruptcy doesn't discharge debts, it just stays some actions on the lender's part until the borrower can catch up.
The only thing the trustee should be doing is making sure the loan is listed in the bankruptcy proceedings. If your brother is playing by the rules, this should have been done. If not, remind him to do so. Other than that, while the bankruptcy is pending the trustee can do nothing (and are required not to bother him about payments).

Once his plan is set in place, he'll make the payments as directed by the court, his "verbal" promises and even what he wants to do means squat. He has to follow the plan.

Are you the actual trustee at this point, or just the successor after mom passes?

There's no "reaffirmation" of debt here, the debt is NOT discharged in chapter 13, just spread out.

Even if he eventually goes chapter 7, if the loan is not repaid (either because of default or discharge), the trust is not obliged to give him property if the trust documents make it contingent on the loan repayment.

I'm confused why your mom set up a irrevocable trust for this.