I know the exact type of cat you speak of. It is very difficult for them to attract the attention they need in order to be adopted.

The best suggestion is to keep him in foster care and allow adopters to meet him at the home.

If this organization does not provide such a service, getting a DIFFERENT rescue group to take him would be a fantastic idea.

Now let me ask you this, would he tolerate a harness and leash? allowing him to go to an adoption event without being in a cage?

Also, would you be willing to consult a vet on the possibility of giving him medication that might calm him down in stressful situations? (this may or may not be enough, for him, but it seems worth an ask)

Best of luck to you and to him.