"Got promoted" vs. "Was promoted"

I prefer "was promoted" and "got a promotion"; but "got promoted" is acceptable, although as already mentioned it sounds much less formal.

The other answers are right that "got" sounds informal in this sense - and it doesn't necessarily carry the implication that he had to work for the promotion (it can even imply the reverse - there is a slight connotation that it happened to him without his involvement, compare "he got mugged").

If you want to emphasise that the promotion was deserved, you might say he earned a promotion, or he obtained a promotion, or possibly even he achieved a promotion. Or you could paraphrase to say something like he was awarded a well-deserved promotion.