At his age and condition I don't think you'll see him signing any more. My grandfather had the condition also, and at that age he was not able to pick up a pen. It's a horrible thing, to have all your senses and be trapped in a body that won't allow you function.
Any way I would suggest going to the large shows like the National I always see them there, or contact other stores. I just got back from vacation and visted this store:
http://www.legendsportsgallery.net
You might want to contact them I think he had some items for sale, there was a ton of memoribilla from a lot of the Legends, it was fun to just look at all the great items.
The owner is a older man, who was very pleasant to talk with, I only wish I had the time to shop more there, he had some great items especially a ton of Bob Feller items.
I found out that Mr Feller is a family friend of his which explained the incredible amount of Feller items.
I know my brother in law has a number of items, he sat next to the Champ on a flight about 15 years ago. It was a cross country flight and Ali handed him a handfull of autos, they all were on Nation of Islam handouts as he was signing them on the plane to hand out to people who approached him. (he's not selling them as he's not a collector but holds onto any personal items he gets, he travels across the US a lot and meets a lot of people on the planes).