Guinea pigs are social animals that enjoy the company of another guinea pig. When a companion is lost, the remaining guinea pig may grieve. Some guinea pigs may seem to do fine on their own but it is always wise to monitor closely for illness and to make sure your pet is eating. Once weight is lost, it can be difficult to put back on. Tracis notes, "Some guinea pigs tend to adjust to being a single pig better than others do. Not eating or drinking, and sitting in one spot continuously are signs that [your guinea pig] isn't adjusting well."
Immediately following the loss of a companion, weigh your remaining guinea pig daily for at least a week. Consider adopting a companion from a rescue or shelter. Quarantine the new animal and make sure your own pet is healthy before introducing them to each other. And spend more time with any pet that is living alone.
On occasion a severely depressed guinea pig can benefit from early introduction to a healthy guinea pig. Rescues may have a stable population and the guinea pig may have been treated for mites and already have been quarantined several weeks to prevent illness.
I usually recommend to people that have a grieving pig after another has died, that they change everything in the pigs life around. New cage setup, new cage location, new feeding times, feeding places, floor areas, floor times, everything. That way, everything is different, and so the pig will be focusing on all the new things that are different, and not just on the one thing that is missing.
You might consider putting the cage in a higher traffic area like the kitchen so he gets more attention. Some guinea pigs may find comfort in having a plush animal companion in the cage.
Thanks to Brandilynn for contributing her picture of Foster.
See also: Companionship and Quarantine