Explaining Alzheimer to a 2.5 YO : Parenting

My husbands family is coming to visit for Christmas. They live far away and our son (2.5) has seen them twice in his life (once he was 8 weeks old tho so that doesn’t really count, the last time was Christmas last year). My husbands mom has Alzheimer. It’s bad, she’s just a shell. She’s barely talking, can barely walk, brabbles incomprehensible words sometimes and doesn’t recognize anyone anymore, including herself. That was last year so it probably only got worse. When we visited them last Christmas, our son was slightly terrified of her. She was a little “cuddly” back then, as in wanted to hold your hands randomly or touch your face while saying things only she understands and that understandably scared my son. We tried telling him that she’s like a baby in her head but he didn’t get it (he was 18 months then) and I’m pretty sure he’s forgotten it. He’s excited for his aunt and uncle to come and recognizes them and his grandmother in pictures so he knows who we’re talking about.

Do I prepare him somehow in advance? Do I just wing it and pretend nothings going on or ignore it? That doesn’t feel right though.

What would you do? He obviously understands a lot more now than he did a year ago. Thanks!