Why this 20 year old muslim boy so fanatic over religion ?

Because he is Muslim. It is their duty to try to p their word on to others. Some Christians do the same thing. If one is complete and happy with him religion and his concept of God it makes sense to want to share it. If you meet somebody and start a conversation and religion comes up this is what happens often. I personally nip it in the bud. But from his perspective he was doing what is right in sharing the word.