No, if he weren't interested he'd have told you to go get your own popcorn.
I was at the movies with a guy, the popcorn didn't taste very salty so he asked if I liked it and if I wanted him to go all the way back and fix it. He paid for the movie and popcorn too. Would someone uninterested, treat you like this?
If he was raised with manners he would have.
Someone who is "uninterested " wouldn't be taking you to a movie .. Period . His asking about the popcorn was a polite gesture .. Don't be too concerned about passing judgement on whether or not he's interested BEFORE your date, that is obvious there is some interest .. It's afterwards that matters. If he is still interested AFTER your date, then you've got something.