A few weeks ago I brought in a rescue Maine Coon, his owner an old lady passed away, and as her only daughter lives abroad, they needed to find the guy a new home, an I have been the lucky one to get him.
Dexter is a 3 year old neutered male, they took him to the parlour and had him shorn, for his hair where matted quite badly. After the shave you could clearly see that he is quite skinny, and you can feel his ribs and spine bones sticking through his skin.
Before his shave.
After his shave
I took him to the vet and had him de wormed, his shots updated, and tested for feline AIDS. The results of the test were negative. He eats quite well, and in addition to his special weight gain pellets from the vet I feed him 3 packets of wet food a day as well.
I really struggle to let him gain weight, do you have any suggestions or do you think he is in mourning, for the loss of his previous owner?
He might be in mourning. I've heard of this before and it's not impossible. I would say that if since you recently took him in and he's not gaining weight, give him some time. Weight isn't going to appear overnight--and also keep in mind some cats are slimmer than others. If after a month he's still like this, see or call the vet again.
Just give him time, and make sure he is eating healthy good quality cat food! My cat was so sick, and I switched the food and he is actually acting like an insane cat right now he is so wired!! If the cat has a healthy appetite this is good, you can just continue what you are doing and make sure the wet food is good quality food. I use Felidae food, its grain free and super good for cats.
If the cat is refusing to eat, as i've said in other threads, I swear by Carlyle's Tuna for cats. My guy lost 5lbs and was skin and bone and not eating. I started giving him this and he got his weight back up, and his appetite back! Make sure, if you make the switch, that you buy tuna for cats, nt supermarket tuna, as cats need Taurine and other vitamins which are not in human tuna.
If he is eating normally then I would say wait for a bit longer. Cats are much like people: they gain and lose weight at different rates. In addition, their "natural" weights are often different. So long as he seems healthy and happy, then it is probably not a major issue. If he does still seem underweight, do take him back to the vet just to be on the safe side. They might recommend something like feeding him in a different pattern (larger portions but less often, for example) or something else. As you mention, it is entirely possible that he is mourning. Not only has his previous owner died, he is also having to deal with lots of other changes (moving to a new house, a new owner, a change of food, etc). All of this is, of course, quite stressful for him so that might be affecting both his weight gain and eating habits. If it is still possible, it might be worth finding out if his previous owner gave him any treats or foods he particularly enjoyed. It might help both with gaining weight and with the transition. In many ways, animals and people are really very similar.
If your cat is eating normally and it seems like he is then don't worry. I agree that in time he'll gain weight. Perhaps he is still recovering from the stress of all the changes in his life that is why even if it eats well it is still not gaining weight. Also ask the VET if the breed of the cat is naturally slim.
Hm. This certainly does sound odd. I know that recently my cat lost a LOT of weight, because she was really sick. As she got better she began gaining her weight back. Do you feed your cat a lot of tuna? I think that's what caused my cats sickness(complete and total speculation, though), so perhaps your cat's getting sick and that's why he's losing so much weight?
How to help my cat gain weight? 
