بسم الله الرحمن الرحيم
(Fatwa: 620/575/M=6/1439)
He should break his oath and pay the kaffarah. The kaffarah of oath is that ten poor persons are fed two times full stomach or they are given clothes. In case he is not able to do so then he should keep fast for three continuous days.
فَكَفَّارَتُهُ إِطْعَامُ عَشَرَةِ مَسَاكِينَ مِنْ أَوْسَطِ مَا تُطْعِمُونَ أَهْلِيكُمْ أَوْ كِسْوَتُهُمْ أَوْ تَحْرِيرُ رَقَبَةٍ فَمَن لَّمْ يَجِدْ فَصِيَامُ ثَلاَثَةِ أَيَّامٍ ذَلِكَ كَفَّارَةُ أَيْمَانِكُمْ إِذَا حَلَفْتُمْ [المائدة : 89]
Allah knows Best!
Darul Ifta,
Darul Uloom Deoband