I would back him up on that assumption 100%.
Well, if you're going to back him, let's see what you've got. Don't be distracted by the words "prejudice" and "elitist". Those will just put you on the defensive.
To prove that all spin-off lit is inferior to all non-spin-off lit, you'll need to find the worst non-spin-off and compare it to the best spin-off. Frankly, I don't think you can.
So let's see you do that. Or else, just concede, because your debate opponent is very skilled and will make mincemeat out of you, especially if you let him get you on the defensive.