My questions are
1. I am not ready to give trees to him now.
This property belongs to your father, hence he only can decide about it.
If he is not willing to give the trees, then he can refuse. The agreement was made long back, but the buyer did not take any steps to enforce the agreement till date, hence the agreement becomes infructuous automatically one it is barred by limitation.
2. Whether the validity of agreement is 3 years (in agreement validity is not mentioned)
The limitation for such agreement is three years if there is no specific period mentioned in the agreement. After completion of three years becomes invalid or noneffective and unenforceable in law.
3. I need to give all expenses beard by him ?(In agreement it is mentioned)
If there is no agreement then there is no expenses to be reimbursed.
4. Not only my father give power of attorney other planter also given to file case against govt in 2003
The power of attorney was given to fight the legal case alone, this will not have an impact to the sale agreement.
Moreover the agreement was not registered hence it is not enforceable even if the buyer prefers to approach court for this.