It could be a slight sprain, but it’s more likely he stepped on something. I would check the paw carefully for injuries or something lodged between his toes.
Other than that I would watch him for a few days and try to keep him resting and if he is still painful on Monday I would take him to your local vet.
i believe the virus is active in the environment for 3-7 weeks. i’d get him checked out before you bring home another cat, and discuss prevention with your vet.