Hi
God bless you for having a burden for your friend, I agree that fasting and prayer are your first and most important steps. I'll assume that you are confident that your friend is actually saved, so then you can ask God to reveal to you if there is any persistant sin in the man's life and if so attempt to restore him gently. Reassure him that the Lord wants him restored rather than condemned.
Galatians 6:1
Brothers, if someone is caught in a sin, you who are spiritual should restore him gently. But watch yourself, or you also may be tempted.
James 5:16
Therefore confess your sins to each other and pray for each other so that you may be healed. The prayer of a righteous man is powerful and effective.
1 John 1:9
If we confess our sins, he is faithful and just and will forgive us our sins and purify us from all unrighteousness.
You probably hit the nail on the head by pointing out that he has placed his confidence in his own feelings rather than on the truth in the Word. I would speak to him about the faithfulness of God, who remains faithful even if we are not. God does not push one of his children away because they have disappointed him or not read their Bible often enough, and not feeling His presence does not change the promises of scripture. The Lord is able to save completely and forever and He placed a seal on us!
2 Timothy 2:13
13 if we are faithless,he will remain faithful, for he cannot disown himself.
John 6:37
37 All that the Father gives me will come to me, and whoever comes to me I will never drive away.
Hebrews 7:25
25 Therefore he is able to save completely[a] those who come to God through him, because he always lives to intercede for them.
Footnotes: Hebrews 7:25 Or forever
Ephesians 1:13-14
13 And you also were included in Christ when you heard the word of truth, the gospel of your salvation. Having believed, you were marked in him with a seal, the promised Holy Spirit, 14 who is a deposit guaranteeing our inheritance until the redemption of those who are God’s possession—to the praise of his glory.
This does not sound like the Lord might just drop one of His children because they have not been diligent or dabbled in the things of the world. To be sure, we can grieve Him and ought to live obedient and holy lives but God does not just quickly withdraw salvation if we fail in that. Unless a once saved person deliberatly turns away and rejects the Holy Spirit's testimony about the love, work and nature of Christ, and so blasphemes Him, one does not simply overnight lose his salvation(at least that's my understanding, there may be other views).
So then I would encourage your friend to stay in the Word, to pray faithfully for God's mercy and to be strengthened in his faith, whether he feels God's presence or not, and to let the Lord search his heart and cleanse Him from all unrighteusness. What is reqired of your friend is perseverance, and the enemy who attacks him with fear will have to leave him alone.
Hebrews 12:2
Let us fix our eyes on Jesus, the author and perfecter of our faith, who for the joy set before him endured the cross, scorning its shame, and sat down at the right hand of the throne of God.
1 John 1:9
If we confess our sins, he is faithful and just and will forgive us our sins and purify us from all unrighteousness.
James 4:7
Submit yourselves, then, to God. Resist the devil, and he will flee from you.
If the fear really paralizes him perhaps you could help him come up with some prayers taken from scripture, such as Psalms. Hope this helps in some way.