Stray has moved in but he hates us!

by L.A. Lady
(California)

We were feeding an obviously stray, beautiful, seal-point Siamese cat for several months (we had papered the neighborhood with "found" notices but no replies). We were amazed he lasted so long outdoors as there are coyotes and wild dogs that come right up to the front door! Plus he has a lame front leg...anyway:

Three weeks ago he came in the house - he is fascinated with my indoor cat (they get along fine) - and hasn't left. He's never even gone near the front door.

He doesn't hide; he even lolls around in the middle of the floor when we have company. He plays with toys. He sleeps on my bed with my cat. We never had to introduce him to the litter box. In short, he acts like he's always been indoors.

The trouble is, when anybody moves, he runs out of the way and hisses if you approach him. He will take treats and lick whipped cream from our fingers, but we've never been able to touch him.

We (with great difficulty) managed to chase him into a carrier yesterday so that he could be neutered, but the vet couldn't examine him due to "extremely aggressive behavior".

We think he's about a year old. Is there some technique we can use to civilize him? Is this typical Siamese personality? Is there any hope for this cat?