Hello and thanks for trusting me to help you and Fred today. I am a licensed veterinarian with over 25 years experience and would be happy work with you.
There are some cats that are true "scaredy cats" and with these guys it doesn't take very much to spook them. There is not much you can do to speed up the process and time is the only remedy. It was probably not the best timing to introduce the new cat, but I guess since he is isolating himself anyway, it is a way to tackle both challenges at once.
Normally, when introducing a new cat to the household, it is best to give him his "own room" for a short while with his own litter and feeding area in the room. There are multiple reasons for this, including making sure he is not incubating any of the contagious cat viruses, like feline leukemia and/or feline aids. From a behavior standpoint, it can be a little less threatening to the other cats in the house if his living space is somewhat restricted to start versus being allowed to have full run of the house right from the start. Cats do not really have a hierarchy in the household, like dogs do, but they are somewhat territorial and may be upset if all of a sudden someone takes over their favorite spot or favorite litterbox. Since Fred is choosing to be the confined cat, this puts a spin on things, but I would still give the new cat his own room for a while until things settle down. This will help Fred to feel more comfortable about coming out.
Even when confining the new addition, Fred will know he is there and he will be able to get used to his sounds and smells through the door, before he is allowed to mingle full force. Even when you start letting them mingle, I would do it slowly and only when you are there to observe at first. If they are left all out on their own, there may be altercations that occur that you are not even aware of. Depending on what the set up of your house is like and where the cats like to hang out, maybe you could alternately confine them in specific areas of the house and let him roam around some also. Eventually, the goal is to allow everyone to be out at all times, but it is often best to build up to that gradually.
Most cats do not like to live with one another right off the bat so we have to strive to provide an environment where all cats are able to find a place where they feel "safe". That begins by creating an environment of "plenty." There should be plenty of litter boxes, food bowls, climbing towers, toys and resting areas in multiple locations. All the litter boxes and food bowls cannot be clumped all in one place because that forces the cats together, something they don't want to do. If that is not possible due to the layout of your home, another option is to create a time sharing plan. One cat is out in the house for a bit, then put into a room and other is then allowed out.
One other thing I would suggest is a product called Feliway®. Feliway is a synthetic pheromone that mimics the natural chemicals that a cat secretes. It creates a comforting, reassuring feeling that has a calming effect. It is calming to cats in stressful situations such as transport, hospitalization, veterinarian visits, boarding, new environments, pets or people. Feliway® is a product that can be sprayed or used as a room diffuser. It can be purchased through veterinarians and pet stores. In your case, a few room diffusers may help low the tension. LINK HERE
I am also including a link to a website about environmental enrichment for the cats. The Ohio State University has composed this to help cat owners structure the environment to provide adequate physical and mental stimulation. There are numerous medical conditions in cats that are precipitated by stress and multicat households are often those with more stress, even if not perceived by the humans in the house. Hopefully there are some ideas there that you can incorporate to help keep the stress levels under control
OSU Indoor Pet Initiative
You really just need to give Fred some time and space to adjust to this all. Getting over the scare related to your husband's fall and now a new addition. He needs a "safe place' he knows he can retreat when he wants where he can feel less threatened. How long this will take is really up to him. Any attempt to rush it will only set things back. Keep doing what you are doing by spending one on one time with him when he wants and have patience.
I hope this is helpful. Please let me know if you have ANY other questions. My goal is to give you 100% satisfaction and if you are not yet satisfied, please reply so I can clarify for you.