How to help my cat gain weight?

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?