Hello, my name is***** and I have over 20 years of experience as a veterinarian. Thank you for agreeing to adopt an older fellow that very much needed a loving home.
It sounds like he is doing very well given his age and the abrupt change in his circumstances. Two weeks is a very short time period for an older fellow to adapt. The fact that he enjoys your company so much, allowing you to rub his tummy and will sit on your lap, is very, very encouraging.
Being an older fellow though it makes sense that he is moving cautious into unknown territory.
Do you know the circumstances of his previous home?
Were there other cats? If so perhaps he was the submissive fellow and the other cats bullied him into staying in one area.
Perhaps he wasn't allowed by the owner to roam that home freely and is behaving respectfully ***** ***** as well.
It is important to make sure that the areas he is allowed to roam in smell like him and have familiar things so he knows he is welcome. Perhaps alternating beds and putting one he has used for a while out in your home will tempt him to come out more.
Make sure he has comfortable, safe perches and places to observe outside of his "safe spot". Perhaps a kitty condo near a window so he can watch the birds will tempt him out. You can put treats out around the house too for him to find, but only have them out during the day.
If your home is a busy one perhaps he is intimidated by that and so keeps to his quiet area until things settle done and he feels safe to explore. If that is the case playing soft music all the time may mask some of the ups and down of the noise and get him feeling more comfortable.
I am glad to see that you are using Feliway. Make sure to spray it anywhere he is allowed to go.
You might also consider a homeopathic product called Bach's Rescue Remedy. It is added to the food or water and helps with anxiety.
Do continue to play with and make a fuss over him in his safe spot. The more attached he becomes to you the more likely he will be to come out and look for you to spend time with.
But I do think it time that Sonny will learn to come out more, he is still probably trying to process a whole new world.
If he hasn't had a physical and some screening blood tests done that would be a good idea too. We do want to make sure that he is healthy and there isn't a vision or underlying arthritis that is slowing him down.
Best of luck with your fellow, please let me know if you have any further questions.