how to make new cat feel comfortable

We rescued a long haired siamese/balinese cat from a shelter this weekend. He is very nice looking, but very shy. At the shelter he was playing lots with the other cats but shyed away from people.

He is 2 or 3 years old, male, fixed

We kept him in his own room with amenities for a couple of days, visiting him a few times a day while he got used to the smells and sounds of our house. He always cowered into a corner or under the lid of a litterbox

We've done some introductions to our 3 year old girlcat Cleo, there was a few hisses but no fighting or anything. Milo (the new cat) is pretty oblivious to Cleo so far.
We've opened the door to his room to let him roam around the house, for almost a full day he just stayed in his corner, but now he is coming out on his own. Still slinking a lot though, and very shy. He likes to hide behind the couches, or in corners.

We gave him a nice gentle brushing a few times yesterday, and after the initial wariness he started purring a lot. He still slunk off afterward though.

He seems to be eating and drinking and using the cat box (his own, not shared) properly

Is there anything we can do to help him feel more comfortable in his new home?

It might just take time and patience. I'd recommend sitting in his room with him and doing small busywork such as bills, reading, etc. Maybe put a treat on the floor near you to lure him over. He might feel more comfortable if you are relaxed and not paying much attention to him. Once he gets used to you he may feel more comfortable exploring because the rest of the house will have your scent on it.

Make sure he has places to hide. A cat carrier with a comfy towel, a paper bag, a cardboard box, etc.

How long was he at the shelter? Sometimes going from a small cage to house is a big shock that requires some time to get over.

He was at the shelter for approx 1 year. But the other cats there are very friendly. He wont look any of us in the eyes and just cowers. He must have been abused or something before the shelter. He wont come out anywhere we are....

He may feel totally comfortable with other cats and not with humans due to being abused or not well socialized. He's also in a totally new environment with new smells, sounds, and people. Nothing is familiar and he's scared. His behavior is totally normal.

Try using a Comfort Zone w/ Feliway plug in at all times.
Go to where he is and sit on the floor. Read aloud to him. Don't look him in the eye, don't try to touch him, don't try to pick him up. Continue doing this until he comes to you.
When he starts coming to you, don't make any sudden movements, just let him explore. Continue talking to him. Don't try to pet him, don't try to touch him or pick him up.
When he starts coming near you, entice him with a string toy or a toy on a wand. DaBird is a great toy to invest in.
Give him time, be extremely patient, and don't expect much of him. That way, when he does become more socialable it'll be exciting for you.

Stephanie