Denying that you are a Christian is a grave sin. If the man committed the act with adequate knowledge and consent then it became a mortal sin.
Given the suddenness with which he was thrust into a situation of grave fear, it is not a certainty that he had adequate present to mind to perform an authentic human act and so the sin may not have been mortal. Still, this is not to be counted upon.
The man needs to go to confession and deal with his action there, just as people who denied the faith during the Age of Persecutions (A.D. 30-311) needd to do so once a persecution had passed.

Puzzled, you're right. It does seem that Christ's words, "he who denies me before men will be denied before my Father in heaven," preclude the possibility of salvation for that man. What do you think, Jimmy?

Wait...Belisarius faught the Vandals, who were highly pagan. Also, the Muslims aren't really his fault.

It is troubling, because "he who denies me before men will be denied before my Father in heaven"
Of course, that sort of belief is why Belisarius devestated North Africa, paving the way for the Muslim conquest of those Christian lands.