There could be lot of reasons for acute Nervous Breakdown
or Mental breakdown to be precise.It could vary from marital issues to issues relating to relationships.Problematic situations at workplace or dealing finance could also cater Mental breakdowns.When a person is not able to fulfill his actual social role a nervous breakdown could easily happen.Remaining in a stressful situation for long time could also be a reason for this.
However,this could be easily resolved if you try to understand the actual reason(stressor) for the nervousness
It is suggested to take your brother-in-law to an experienced psychologist
who will help him to unwind and understand the actual stressor.
And regarding his speaking like a kid, could just be one of the hysterical episodes that come in such nervous breakdown.Before it gets worsened to a Mixed anxiety
and depressive disorder, it is advised to take him to a clinical psychologist.Treatments like Cognitive behavior Therapy could easily help in resolving his nervousness.