I'm bisexual, and not much older than your son. If he thinks he's bi, he probably is, but I'm sure he'll tell the truth eventually if it was just a fling, or a bit of confusion. He doesn't- I repeat DOES NOT -need a therapist! Just support and love him unconditionally, and it'll be fine. Telling him it's 'wrong' will only make him pull away from you, and that's never good. He needs a strong, compassionate parental figure in his life right now.
I can understand why you would react to the hickie, but it's just something teens do, hetero or homo. Even if you don't permit sleepovers, he may come home from school with a hickie, or after hanging out with his friends. It's unavoidable, really. But don't worry about it too much, it's normal.
Start with asking him about when and why he started to have homosexual feelings, and just talk to him. Listen.View Thread