Congratulations!!!! That is so wonderful!!! It will take a very long time for your Simon to adjust. You just have to be patient, which I know you are.
We adopted our 2 boys from the shelter when they were about 6 months old. The one cat is as friendly as can be and took to us right away. The other one was scared to death. We kept them separate for about a week or two and the one who was so scared stayed under our bed for a week!! It was awful. I felt so bad for him, but we just kept the door closed when we weren't in there and put the litter pan in with him and food/water and went and sat in the room and talked with him all the time and eventually he came out. Mind you, he was a kitten and this scared. So I can only imagine how an adult cat must be extremely terrified to be removed from the only home he's ever known.
He'll calm down in time and will know you love him and that's all that matters. Just keep him confined until he doesn't run under the bed everytime you come in the room and slowly introduce him to the family. Everything will work out just fine. And try not to make any sudden moves or loud noises at first. I had never seen a cat jump so high in my life!
I'm so happy for you! I just love hearing about a new addition, it gives me goosebumps!