If your cavy is acting lethargic at all, this could be a sign of something going wrong. Often, small animals hide symptoms (survival thing, you do not want to let on that you are sick – you become easier prey). If he had been active at the pet store and even at home and that is changing, he should be seen by a vet to rule out medical issues first. Best to play it safe as often it is the subtle changes we miss. If he is healthy, then he may be craving attention. Cavies are very social animals and live in groups. Isolation for a cavy is not fair and against what they are. You may have to step in and provide that extra stimulation. Now, cavies from pet shops are mill bred just like puppies and kittens. There is no care taken for health and temperament. (I used to personally know a couple mill breeders and would never recommend a pet shop animal for the reasons I have seen). Also, it is very hard for these animals to recieve proper socializing as youngsters so that they learn to trust and bond to humans. If you decide to ass a second cavy, please, go to a rescue and choose a good match for your boy (just make sure if you get a female that he is neutered first). A good rescue can help you pick a good match. But rule out medical first. There also may be nutritional issues going on, etc.