Since you've had him four months already, and this started three months ago, it would not be my first guess that he's missing his housemates - although it could well be that. Anytime there's a sudden behavior change in a cat that can't be explained, my first inclination is that there may be something physically wrong with him. It might not be a bad idea to have a vet visit to rule out an illness. If he checks out, then start looking for other causes.