Paid leave can be earned leave, sick leave, or casual leave. If your establishment is a factory, for the first year of service, the employee does not have any leave. His leave will accrue after 12 months of service. For convenience, prorata leave will be credited in January based on the days worked in the preceding year, i.e., the current year, say 2021. This leave is called annual leave with wages or earned leave. Other than earned leave, there is no leave available. As such, he cannot demand leave for the past 6 months.
If your establishment is a shop, obviously, depending upon the state rules, there can be casual leave. In my understanding, the maximum number of casual leave is 12 days. That can be allowed only on a pro-rata basis. Again, under the Shops and Commercial Establishments Act, privileged/earned leave accrues only after completion of twelve months and not in six months or completion of probation.