Think about the fact that before Jesus came and died on the cross., the veil was not "rent in two"., the Holy Spirit was not yet given to seal believers., The covenant of law was replaced by the covenant of grace. The disciples were not all saved. We have to be educated as Christians that there is a major difference before the cross and after the cross. Everything changed.
I'm sorry to hear about your nephew. But I also think that happens a lot to believers. We are frail people in our flesh especially when we are not abiding on the Vine. We are devoured by the enemy when we don't know the truth. The devil counts on us being overcome by our human reasoning power.
Did you ever notice that psychiatrists have a high rate of suicide? It's because they think they understand life and human behavior. So when they see none of it matters anyway no matter how much they get good at life and understanding it., what good does it do in the end? They die just like the bad guy and the wacky guy. Worldly smart people don't factor in Jesus Christ at all. We can have all the understanding of human behavior but miss the truth without Jesus as the way on the path of life.
Many believers suffer from depression and anxiety. Many are on medications. (many are sick and weak and many sleep) There are lot's of reasons for this but one of the reasons is; they don't know or believe God loves them unconditionally. And if they don't get good teaching after they are saved they will be open to the fiery arrows of the enemy. he comes to kill., steal and destroy us. Many Christians commit suicide because they don't know the truth. The Bible says my people die for lack of knowledge.
We've been warned about our enemy's schemes but I've discovered in my own life too that he has had years to use our own minds and emotions against us weak humans. And if we don't know the truth that Jesus came to save us and bring us out of death into life making us new creations., never leaving us or forsaking us., giving us a new nature and a new way of life, we can fall into devastation by what we see in this life and how we feel about what we see and know.
So it doesn't surprise me your nephew and many other Christians are disappointed in Jesus and what they used to believe about Him. There is more your nephew and others like him need to learn about Jesus but they have been derailed in their faith and are still running. The Bible says even if a sheep falls and goes astray the Good Shepherd will get them. This life is a tiny portion of what is actually going on. Living life as a Christian and living well as a Christian with joy and peace are 2 very different things. Many believers are bitter angry people who are at the root of their lives afraid. Fear is a spirit and the Bible says God has not given us the spirit of fear... But He has given us power and love and a sound mind. We each have to access the truth of that and what it means for each one of us personally.