I have known so many little boys with "obsessions" for things....like fire trucks, or trains, or dinosaurs! The problem with your lo's obsession is that there are elevators everywhere, which isn't the case with some of those other things! So he sees them all the time and they are so accessible that he wants them all the time!
If it were me, I would designate a time, say, Sunday afternoons, to go to the mall, and ride that elevator a set number of times, say....20 times, or a block of time, say, 15 minutes. I would wrap that trip in other errands, (do some shopping while there, etc) and that way he has that to look forward to all week. So then when he says he wants to ride, you can say "Sunday is lift day, we will do that on Sunday". He might still get angry, but in time, with Sunday (or Friday or whatever) being "Lift Day", he might settle in to that routine. I would make a good time of it if this were me, ride the lift, get an ice cream, go back to the lift, etc.....and then maybe his cup would be filled. ?