Dear Ajith,
You have opted for composition scheme, hence you are unable to take credit. If you had not opted for composition scheme, then you could have taken the credit and paid service tax at the standard rate of tax.
If you have made the first service tax payment in respect of this project at the composition rate, then you cannot change it for the said project/contract. Hope you would have assessed the benefits of opting for composition scheme.
In the given scenario, you have to pay the service tax demanded by the sub-contractor.
Thanks,
Ganesh