Is it bad if one guinea pig is living by himself?

Guinea pigs are a social animal and do better around their own kind. Boredom and solitude can make a single piggie depressed, but that can be overcome by spending lots of time with your pet and having plenty of toys and things to do when you are not around.
My single female piggie was just fine, but she had a big cage, lots of toys and tunnels and I would spend at least 3 hours a day being social with her and let her explore around.
I will say if you can accommodate a playmate, they do better though.

Yes, they are social animals and need company, get another one of the same sex, or neutered

Yes. Gunnie pigs need friends to communicate with. It would be like you living alone n never being able to talk to anyone but yourself. It would be lonely n depressing.

No :) as along as you keep him entertained :)