Actual renunciation is irrevocable, which means that you can't un-renounce, though an act described in 8 USC 1481 can be challenged administratively challenged with State through a lawsuit: here is the manual on the former option. Taking citizenship in Canada for employment reasons (in the era when certain jobs had citizenship requirements) is not voluntary renunciation, and can be administratively undone. See Vance v. Terrazas for the controlling case law on this: what the government has to prove is that the person intended to relinquish the rights and privileges of U.S. citizenship (typically not the case with employment-related other-citizenships).
Assume that he actually renounced citizenship, for a non-tax reason, appearing and signing the correct oath of renunciation before a competent official abroad at a US embassy or consulate. He is now not a citizen: but there is no attainder on his soul, he can apply for admission to the US just like any other non-citizen can. Gaining citizenship by ordinary means is not the same as reversing a renunciation of citizenship.