Other friends advised me it was because he had always wanted more with me and once he was involved he moved on.
I had that happen to me, except he gave me a big "It's your fault I can't talk to you anymore" speech before he cut me off. I was fucking wounded, but I needed it spelt out for me that it wasn't my fault, and any guy who suddenly can't hang with you because he has a girlfriend (who's a mutual friend) was either looking at the friendship differently from you, or the girlfriend is exercising some leverage. Because I couldn't have told him more often that I didn't want to sleep with him (or anyone). I was adamant about that.
Laura, I cannot imagine the stress you're going through with your son. You're incredibly philosophical and reasonable about the whole thing, and I can't imagine being all that in that scenario. Go you, and here's to him pulling his head out of his nether regions sooner rather than later.