It is sad to hear these things. Sometimes a brother stumbles because something of the world gets his attention and draws his love and vision from Christ. Sometimes a young Christian may become discouraged because he does not see his more mature brothers and sisters in Christ fully living up to their profession or not retaining vehement love for the Lord Jesus. There could be a host of reasons, but I believe that the following would be a Biblical approach to dealing with such a brother:
Step 1: pray. "If any man see his brother sin a sin which is not unto death, he shall ask, and he shall give him life for them that sin not unto death. There is a sin unto death: I do not say that he shall pray for it. All unrighteousness is sin: and there is a sin not unto death." (1John 5:16-17)
The value of prayer by other Christians who are walking with God cannot be too much emphasized. James 5:16 says, "Confess your faults one to another and pray one for another, that ye may be healed. The effectual fervent prayer of a righteous man availeth much." There are a number of great men from the past who wandered from God for a time or grew spiritually cold who were eventually restored and mightily used of God. I can think right now of one specific instance, although I'm sure there are many, in which that man later stated that it was the prayers of a mother, sister, etc. that brought him back to the Lord. If you are married, you might mention the matter to your husband, and it would be good to mention it to your pastor as well, so that a number of you could be praying for this young man. If you know of some other discerning, Spirit-filled Christians who could pray for him as well, you should be very discreet in mentioning this burden to them, so that it remains a matter of prayer without taking on the appearance of gossip.
Step 2: Confront. "Brethren, if any of you do err from the truth, and one convert him; Let him know, that he which converteth the sinner from the error of his way shall save a soul from death, and shall hide a multitude of sins." (James 5:19,20)
"Brethren, if a man be overtaken in a fault, ye which are spiritual, restore such an one in the spirit of meekness; considering thyself, lest thou also be tempted. Bear ye one another's burdens, and so fulfil the law of Christ." (Galatians 6:1-2)
As a lady, you may not be the best person to confront this young man. The pastor may be the best one to do this. It will certainly need to be someone with spiritual wisdom for at least two reasons. First, because they will need to be able to offer guidance in what appears to be a very dangerous situation (and will have great need to be in touch with the Lord). Second, they have to be a person that will not be a stumblingblock to this young man. It can be difficult to accept guidance, even if it is sound, from someone whose character contains something you do not respect.
The direct approach would probably be best. If this young man was indeed walking with God but has grown cold, it did not likely happen overnight. He will likely know what the Lord had been convicting him about and, therefore, what it was that turned him out of the way. It may be that, like David, he simply needs a Nathan to tell him, "Thou art the man!"
It must be done in the spirit of meekness, both for the sake of this young man, and also for the sake of the one who confronts him. A meek spirit will often prevail with someone where a self-righteous attitude will utterly turn them away. Also, we are not above being tempted in a sin from which we are trying to turn someone else, nor are we above being tempted with pride when we seek to lift up another. I can remember a time that I tried to help someone else and made an unwise agreement with them which brought much damage spiritually to myself and did not seem to help them.
Step 3: Pray again. It may be proper to fast as well. I believe it was Justin Martyr (c. 150 A.D.) who said that when one wanted to seek the Lord and join the church in his day, the congregation would fast and pray for that individual for three days. When an eternal soul is at stake, prayer is so important.
Even if it takes a long time, it could mean a world of good for you not to give up on this young man. Try to remember that Christian charity "suffereth long and is kind" and not to be "weary in well doing." Some are reclaimed even after years of wandering.
May the Lord bless you and be powerfully present to draw this young man to Himself.
--Charles