(2) has no implication at all about how many brothers he has. If he has a single brother and his name is John, that's his brother John; it's also his brother, John. In (1) there is a fairly clear implication that he has only the one brother, and it's naming him as John, but even this is not definite. If he has two brothers John and George, I can certainly say that his brother is coming over for a visit. Of course, if you know he has two brothers, I might need to explain which is visiting: so his brother - John, I mean - is visiting. Thus his brother, John, is visiting. This is comparatively unlikely but still possible.