Is it illegal to secretly take a screenshot of someone's WhatsApp chat on their phone and post it online without their consent using your own mobile?

Yes, you can pursue legal action to claim the arrears in maintenance for your son. Indian law mandates that parental support for a child continues regardless of the parent’s marital status or the child’s living arrangements. You can approach a lawyer to file a case for recovery of the maintenance amount owed, potentially with legal aid assistance.