10 week old kitten seems depressed or maybe a little stressed?

My Siamese had a litter of four kittens, two of which have sold recently and gone to good homes. All kittens have been wormed, vaccinated and vet checked. One of the two kittens remaining is thriving but the other one has had a small case of diarrhoea (has settled now) and he seems to have stopped putting on any weight. He hasn't lost his appetite (raw chicken and roo meat). He is still running around but not as much since he lost two of his siblings. Could he be depressed or possibly stressed from the change and what can I do to help him put back on the weight?