How can child ask for share in ex husband's property? What procedure?

Yes, your child has the right to claim his share in the ancestral property of your husband. This may be done by filing a suit for Declaration, Partition and Possession. Your son will get his share once the Decree for the same is awarded. This is irrespective of the age attained by your Son at the time of the Decree.

However the true position can only be confirmed only after examining the Divorce decree and the orders made there under with reference to the maintenance of child as we need to understand what all maintenance was seeked in your petition and what all was awarded.