Aww! And let me just say, I adore the name Simon, it's my son's name
But yeah, just give that sweet baby some time to adjust. He'll definitely come out when he's hungry or thirsty but it may still take some time for him to get used to the new family. My mom and dad had a cat who hid out for like a month or 2, but, she finally came around and she was the sweetest little cat! It was almost too much, actually, everywhere ya went, she HAD to be right there! lol
Good luck with your new babies!