I don't have any experience with St. John's wort, but I would discuss it with your son's doctor first (to avoid any interactions.)
As far as feeling guilty about
not seeing your son's depression earlier, I think we will always feel that way somewhat when it comes to our children. When you are so close to someone it is sometimes hard to see the subtle changes over time.
Thoughts and prayers are with you and your son,
Jon, Co-moderator for Crohn's Disease, Depression, & Celiac Disease forums
"The man who insists upon seeing with perfect clearness before he decides, never decides. Accept life, and you must accept regret." -- Henri-Frédéric Amiel (1821-81), Swiss philosopher, poet