It's difficult to convert visitor visa to any other type of visa from US. There are possibilities for extension for next 6 month if he has good reason for his stay. If he still wants to apply for change of status, no one can stop him legally, but be advised that only exceptional cases get approved.... rather I would suggest to get I- 20 and go back to Nepal or any other US consulate and then request for F1 visa stamping, this will provide him lots of freedom to visit Nepal and other countries in future.