Similar situation here, especially with regards to school not acting. Have been advised to back-up our daily diary of all phone-calls, appointments and emails/letters via daily e-Mail, which, in the country we are in, is legally admissible in court. Document absolutely everything - appointments, phone-calls, chance meetings, anything he remembers that the kids have said or done, no matter when he thinks of it.
Also, trust no-one and expect nothing from anyone. Someone expressing doubt of the abuse has a domino effect, at least in our experience. The amount of people who have denied our child's abuse is overwhelming, and very often the people we thought we could trust have proven untrustworthy, even parents who have also reported their own child's abuse.
When your DB is not satisfied with the answers, or getting no answers or help from people he should be, go higher. Get their bosses or their bosses bosses. Make a lot of noise, because child abuse is still covered over, and people will not acknowledge it.
Send every reports to all people involved. Link everyone - the school, child-safety, doctors, psych. Give them permission to talk to each other, because that, for us, is the only way we have had any support and made progress.
Lastly, support his mental health. Make him food, take him our and distract him, and get him to a doctor if he is not sleeping or eating or getting depressed. Medication is not to be feared - it is an acute and extremely stressful time, and he needs a lot of endurance to get through it.
Remind him that you believe him and support him. Be patient. And be kind to yourself, too.
I am so proud of your brother for doing this. Please, let him know he is not alone, and that we, amongst many, many others, have fought and are still fighting for our children. Take courage. All the very very best.