It is completely normal. When I was in high school, I would somtimes get an erection when my teacher (a Christian Brother) would pass by my desk. It happened that another "Christian" brother was the 1st one that abused me before the priests and my sperm provider got hold of me and made me their "play-toy" for the summer. Was this caused by the abuse? I'll never know but think it was.
Since he is talking to you, this strongly implys that he trusts you greatly. Remember that if he finds out that you violated this trust, it will set him back to the start. If you want to help him, ensure that you look after yourself in all ways, mentally, physically and good nourishment.
I will mourn the teenager I never was and strive to make that dot of light way out in the far reaches of the end of the tunnel turn into a bright sun.
WE ARE NOT VICTIMS. WE ARE THE SURVIVORS!!!