Hi Ali, currently there is no native CLI option to modify the TFTP timeout.
An option would be creating a Python script to ping the server through MGMT vlan and only start the TFTP process if the server is up.
However this script might need to be created from scratch.
You can find some examples for Python scripts in the link below:
EXOS Python Scripts
Note: Python version 2.7.3 has been integrated into EXOS since EXOS 15.6.1