He should busy himself with reciting the Qur'an, seeking forgiveness the Prophet (sallallahu alaihi wa sallam) said in hadeeth reported in Bukhaari on the authority of Abu Hurairah, "verily, I seek forgiveness from Allah, and I repent to Him more than seventy times a day" and in the hadeeth of Ibn Umar, the Prophet (sallallahu alaihi wa sallam) used to say a lot, "O Allah forgive me and accept my repentance, for You are At Tawwaab (ever accepts repentance) and the Most Merciful" 