What a cutie. Orange cats are the best, I love them.
When we add a new cat to our gang we keep him in the bathroom for a day or so and let him get used to being in a new place. It's scary to be away from the home you've always known. The small space of the bathroom helps him to be less overwhelmed. We will go in to check on him and spend time letting him know that it's ok. The other cats also "visit" him under the door. They peek under the door and sniff each other. Then when he does come out there is still some hissing but it seems to be a little easier on him.

This is what has worked for me, I hope it'll work for you.
Good luck.