Worsening twitching may indicate that he has a neurological problem, this is also suggested by the fact that he loses his balance. If this continues to be a problem (and worsens) I would highly recommend seeking a second opinion by a neurologist that specializes in animals. In pets with neurological problems, it's better to find out sooner than later if they have problems that can be fixed. For this reason, I would recommend calling your vet and having him make a recommendation to someone that can do a neurological exam on your boy. Hopefully, if something does turn out to be wrong, we can catch it and possibly slow the progression or stop it all together.