He has a clack from the starter solenoid, so it's not the transponder or clutch switch.
The fact that the starter is trying, but does not spin, coupled with complete power loss immediately after, leaves very few options.
It's a bad main connection somewhere not providing enough amps to crank the starter...I'm sure of it.
I'll post a wiring diagram later so that you can see. If the solenoid is activating, and the starter is fine, that leaves only connections. The starter is connected directly to the battery,
via relay, so if it's getting power to the solenoid, then the relay is getting the "OK to start" signal.
Check the negative terminal to ground cable (both ends) and trace the positive side all the way to the relay, then from the relay to the starter. Also verify the relay is good and doesn't have pitted and scorched contacts.
Everything else is a waste of time
Seriously.