必须用现在完成时造句

不对。since当自从...将时，从句中若是延续性动词 ，则从延续性动词结束时算起。也就是说he was a child的结束时间应该是“长大”。
所以，Since he was a child应改为：Since he was a baby.
或者Since he became a child

基本正确

这个人还活着吗？（恕我这么问）
如果不是，用had
He has been a Christian since his childhood.好一些。本来用不着从句的。