Not knowing what model your tractor is I can only tell you what I know about my Kubota. Most small tractors under 40 hp. have a system called OPC. Operator presence control. I had issues and it was all related to a seat switch. The kubota has several switches mainly on the seat, pto, and hydro pedal. They all can cause no start and shutting down. I would do some checking to see what system his tractor has. Also check the fuses as when mine was acting up I accidentally blew the OPC fuse and then it would not crank. Replaced the fuse and then diagnosed it to the one seat switch that was not registering some one in the seat. Mine also had a second seat switch so if you flipped the seat up over the wheel the switch would allow the pto to run with out an operator in the cab. Rear pto implements like a rear 3 point log splitter or chipper. Do some research on that system.