OK the "multiple" part is scary, they need to figure out what's going on to stop the damage. But as far as damage he's already suffered, don't despair! The brain is an amazing thing, amazingly adept at adapting to damaged areas.
Ted had a stroke years ago and lost 20 years of memory. He didn't recognize me or our kids, called them "hey....buddy" and seemed puzzled by these little people hugging him, he thought he lived in another city, he didn't remember things that happened 5 minutes ago. I really thought I'd lost my partner and gained a very mentally disabled dependent. And then slowly but surely, lights started going on in various corners of his brain.
A friend experienced even worse brain trauma and also recovered to an amazing degree.
So you're right to be scared right now, but just don't look at where things are now and imagine it's your future. This may be the "mini" event that clues you in to whatever the underlying problem is, and causes him to get on blood thinners or whatever precautions are appropriate, to prevent a more severe problem in the future.
Know that you've got a lot of people out here pulling for you, and that you will get through this