I have 3 cats: one female (Lucy, about 2 years old) and two males
(Rascal, about 1 year old, and Socks, probably 3 years old).  All were
strays.  Lucy was the first stray and she is very docile.  Rascal has
been with us about 6 mos.  Finally, Socks was taken in about a month
ago.  They are all indoor cats, and all are spayed/neutered.

Rascal is the smallest of all the cats, maybe 7 lbs, compared to Socks,
who's the largest at 15-20 lbs.  Socks isn't fat, he's just big.
Anyway, it has gotten to the point that Socks won't even come out to
use the litterbox or to eat because Rascal constantly chases Socks off.
Socks is extremely timid and won't defend himself.  He will hiss at
Rascal, but if Rascal ignores that and comes at him, Socks will run and
hide.  Usually he hides on one of the chairs at the kitchen table.

This weekend, I had been wondering about Socks using the litterbox.  I
had assumed that he was coming out at night.  Well, yesterday I smelled
something and eventually found his "secret" litterbox: he had been
using an old shirt that had fallen on the floor way behind the kitchen
table where it was not visible.  I immediately stuck a second litterbox
under the table temporarily.

I had a time getting Socks from under the kitchen table.  He ate like a
demon and drank like crazy.  The poor boy must have been so awfully
hungry and thirsty.  Now he's hiding in a corner in the bedroom under
furniture.

A bit of background on Socks: Socks was originally an outdoor cat.  He
hung around outside for at least 6 months.  He was intimidated by two
other stray cats who were also outside, and sometimes I would sit at a
distance and watch Socks eat while keeping other cats at bay from his
food.  I tried everything to get him to come to me, and it took 6
months before he would let me touch him.  After that, he just rubbed
and rubbed and purred like crazy.  Eventually, one of the other cats
disappeared, and the other I had to capture and sign away to a vet
because he was ill and extremely aggressive to me.  Now that I have
taken Socks in, he is such a loving and docile cat with my boyfriend
and I.  He will not play with Rascal, obviously, nor with Lucy, and he
will not go near other humans except for my boyfriend.

Lucy accepts Socks but they do not really associate too much.  Socks
keeps to himself.

How can I keep Rascal from being so aggressive to Socks, and how can I
bring Socks out of his shell?