He could renounce it just as naturalized citizens need to do by oath. A failure to renounce all loyalties except to the US should be grounds for revocation of the US citizenship. Otherwise the Constitutionally created oath of citizenship by naturalization is illogical.
No he can't, that's Panama's decision.
As I noted in my last post Cuba will not accept renunciation of citizenship. Should we strip naturalized Cubans of their US citizenship.
Like it or not the United States cannot revoke citizenship on behalf of another country. The fact that naturalized citizens renounce all other loyalties (that's the first issue addressed in the oath, all naturalized citizens have done it) has no impact on the laws of another country.