I have been rereading Tae Kim's entry on the ～ている form and it mentions how with motion verbs (行く、来る、etc.) it means a completed action, rather than an ongoing one (or rather, an ongoing state of having gone, come, etc.). Is 成る one of these verbs? In other words, does 彼はxxxになっている translate to "He is becoming xxx" or "He has become xxx"?

It means "He has become X", but I wouldn't call it a motion verb. Motion verbs are examples of verbs that can't take the "ongoing action" meaning of ている, but there are other verbs that fit that category as well.

When I was taking classes my teacher called the difference between the two categories 'flap verb' and 'flutter verbs.' 'Flap verbs' are either on or off, completed or not completed, so ている means a completed action. Motion verbs are one type of flap verbs. 'Flutter verbs', on the other hand, can be ongoing.

Note also that a lot of flutter verbs can be used to mean completed actions. 食べている could mean that you're eating, or it could mean that you've finished eating but it's relevant to your current ongoing situation (for example, when explaining that you're not hungry because you've just eaten lunch).