He should seek refuge in Allaah.
أَعُوذُ بِاللَّهِ مِنَ الشَّيْطَانِ الرَّجِيمِ
I seek refuge in Allah from Satan the outcast.
'A'oothu billaahi minash-Shaytaanir-rajeem.
He should renounce that which is causing such doubt  He should say:
آمَنْتُ بِاللَّهِ وَرُسُلِهِ
I believe in Allah and His Messenger. 
'Aamantu billaahi wa Rusulihi.
 Al Bukhaaree with al-Fath 6/336 and Muslim 1/120  Muslim 1/119,120.