Can anyone help with the minimum salaries earned by a foreign student?
Zero. Which is pretty much the same as the maximum you can expect to earn.
What type of jobs do they get?
Basically, none. There are extremely few jobs for foreign students, so make your plans on the very safe assumption that you will not earn a single penny during your stay. Likewise, there is no financial aid available for foreign students, nor can you get student loans from Polish banks.
If you or your family cannot afford to pay EVERYTHING, forget about studying in Poland.
What would be the average expenses per month he would have to face.
500 Euros a month would be a safe minimum, and perhaps more depending on your lifestyle. That does not include tuition, books or supplies.