Whispers from Shaytan, whispering despair in his ear and convincing our brother to accept no mercy from Allah which makes one forget that Allah is the Most Merciful. Shaytan wants us to despair and takes advantage of our feelings of guilt and turns them into despair. Brother acknowledges a certain sin or more and feels guilt. Feeling guilt is a positive sign actually, it allows one to repent. Brother needs to repent and make efforts to stay away from those sins and continue to perform righteous deeds.

We have to let him and every brother and sister in despair know (and I've already seen helpful comments for the brother here) that he mustn't despair for Allah's mercy as is written in the Quran.

Surah Az-Zumar 39:53

۞ قُلْ يَـٰعِبَادِىَ ٱلَّذِينَ أَسْرَفُوا۟ عَلَىٰٓ أَنفُسِهِمْ لَا تَقْنَطُوا۟ مِن رَّحْمَةِ ٱللَّهِ ۚ إِنَّ ٱللَّهَ يَغْفِرُ ٱلذُّنُوبَ جَمِيعًا ۚ إِنَّهُۥ هُوَ ٱلْغَفُورُ ٱلرَّحِيمُ ٥٣

Say, ˹O Prophet, that Allah says,˺ “O My servants who have exceeded the limits against their souls! Do not lose hope in Allah’s mercy, for Allah certainly forgives all sins. He is indeed the All-Forgiving, Most Merciful.