Hello and thanks for trusting me to help you and Monkey today. I am a licensed veterinarian with over 25 years experience and would be happy work with you.
It is always sad to watch a pet grieve and I am glad to hear that he has made progress. Given that however, if he was used to having the company of another cat for all of those years, he would most likely enjoy having another cat around. It is usually the cats that were used to being alone that have a harder time when you introduce a new house mate, but he was already used to sharing space and would more likely adapt and welcome.
Remember that cats are not always "friends" right from the start and so when bringing in a new cat, it is best to give them their "own room" for a short while.There are multiple reasons for this, including making sure they are not incubating any of the contagious cat viruses, like feline leukemia and/or feline aids. If they were not exposed to other cats for awhile previous to coming into your house, they may have already been screened for those however and that would be less of a concern. From a behavior standpoint, it can be a little less threatening to the other cat in the house if their 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 litter box.
Even when confining them, Monkey will know they are there and they will be able to get used to their sounds and smells through the door, before they are 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 Monkey likes 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 may not 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. When they are separated, we can then proceed to limited introductions, where the cats are rewarded for being calm around each other.
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. Even though, once he gets used to a new pet, he will probably enjoy the company, the introduction of a new housemate can be stressful.
OSU Indoor Pet Initiative
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.
My posted replies are for general education only and not meant as a diagnosis. Only after a thorough veterinary examination can a diagnosis for your pet be made and specific treatments be advised or medications be prescribed.