I see no moral imperative to tell people about others financial status unless not knowing would directly harm them, like if they were about to lend money to someone known to not repay their debts.
People don't have a right to know everything about everyone.
In this scenario my sons wealth would not negatively impact those relatives so I have no moral obligation to tell them. In fact, telling them even risk harming my sons relationships and physical and emotinal and social wellbeing, so it's more immoral to tell my relatives about this.