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.
