I asked a senior to help me get an interview since I want to switch my job. I am a little worried though. What if I get rejected and he never helps me out again?

Do people put their reputation on the line when they recommend someone for an interview? What if that person's interview doesn't go well? Will they stop helping?

probably yes, even if it was smooth from your end and you got rejected for no rational reason whatsoever.

Omg I am scared

I don't think if you get rejected, it will look very bad on him. At the end of the day, it is just a recommendation. However, if someone recommends you for an interview, and you chose not to attend it, or after getting an offer, chose not to accept it, that would look bad on him. My take on it- I may be wrong though.

Correct

So if I try my best to study well but I am unable to answer a few questions and the partner doesn't hire me, he may still help me out in the future?

Yeah.