Poor guy! I suspect he's agitated because he knows something happened or is simply looking for his friend. When we had a bonded pair and one was sick and had repeat vet visits, the piggie left at home used to run around frantically looking for her sister, though ironically after the sick one passed away she didn't look for her anymore. But she spent some time in the cage with her after she had passed away... I think on some level she understood that her friend was not coming back that time.
I would give him some extra love and attention, and consider getting him a friend in the near future. My experience is that our remaining pig didn't completely brighten up until we got a companion pig for her, and then she was obviously very excited!