If a student went to Canada on a visitor visa, is he can apply for study visa from there only?
If you want to apply for a course during your visitor visa, you can enroll for a course that has a duration of 6 months or less. For such courses/programs, you do not need a student visa or study permit. In case you are planning to convert your visitor visa into student visa, you can do the same. However, doing so while in the country will bring down your odds to getting it confirmed. It is because one of the most important prerequisites to get a student visa is to show that you will leave before your study permit terminates.
So, applying inside Canada will demonstrate that you are doing whatever it needs to remain longer or not to leave the country. However, the chances of getting a student visa when you are on a visitor visa depend on the documents you submit and details you provide. Lastly, you must keep one important point in your mind is that even if you are applying during your visit to Canada, you will need to arrange everything, right from getting admission to your preferred a university or college, collecting essential documents and funds proof, etc. within the validity of the tourist/visitor visa which is again a critical task.