Please help: Olympus OM-1 stuck...shutter(?), mirror(?), something!

I'm trying to revive my teenage son's OM-1 which just isn't working any more. After taking off the bottom cover, I can use the advance lever and the shutter button depresses, but it doesn't actually move the shutter...it gets stuck.

Here is a video of the mechanisms and what it's doing when I try to use it:


Thanks in advance for any suggestions you can offer.