Having some past success with this type of challenge, I've found that it's a two step process.
The first is building trust, common ground and friendship, if possible. The usual hoarder has been harassed by family, neighbors, city and the county. In spite of all that , he holds on to his goodies. Understand that he has an emotional attachment to his collection. Very few people have ever admired his collection. That's the key. Don't mention buying anything at this point in time but finding out if there's anything he needs is always a good idea. Money doesn't usually have much influence on hoarders. You're there to admire his collection and listen to him share all his plans for everything he has. No need to remind him that five lifetimes wouldn't be enough. He knows that. You will also want to share your projects with him too. He'll start to see your need for parts before you bring it up.
Second step is to get him to understand that he would feel good seeing some of his stash on the road. Use his emotional attachment in your favor. Money may have some influence on this but it usually isn't the deciding factor. He has to FEEL that his parts will be going to a better home than he can provide. He also deep down is afraid that all his stash will be tossed upon his demise and he doesn't want that to happen. The situation is similar to owning a valuable (to me, anyway) part that I don't want to get rid of but if a friend needed it I'd let him have it.
Don't rush either of these two steps. Deal in truth. Trust and friendship must be built up to have success.
I wish I lived closer and could join the "bring out your dead" team. I love looking at collections of "vintage tin" and talking with the collectors. Best of luck.