Trv has extremely high rejection rate in India. Even sowp is difficult to get but the only road ahead is applying for owp showing strong ties to home country.
Once he is on the post graduate work permit, he will have to secure a full time job that is skill level A, B or 0 before you will qualify to apply for an open work permit.
BTW you cannot choose a visa office. It depends on the region/state where you are living. If, for example, you live in any of the south India states/Maharashtra/Gujarat then your visa application will be processed by the Bangalore visa office.
It's not as if a specific visa office has high/low rejection/approval rates. If you prove strong ties then a TRV will be issued