Ugh, I'm so sorry - we've never medicated my son, but we've certainly been there with the depression and threats. (We were considering it, and then we got other intervention that helped; it was a temporary situation.)
How old is your son again?
I'm no doctor, but I've experienced depression in my own life, and based on solely my own experience, I think there are two types of depression: one that I call "functional" that is a response to some situation or some issue that causes deep frustration and sadness. For a lot of kids on the spectrum, I'd imagine there are any number of things that can cause this: difficulty communicating due to pragmatics issues, difficulty managing sensory functions, difficulty tearing oneself away from special interests, etc.
There's another one, that I call "residual" depression, I think this is what people mean when they refer to a "chemical imbalance." It's often preceded by functional depression, but it's more about one's brain getting stuck in a rut and not being able to shake the feelings of deep frustration and sadness. This has little to do with what's going on in someone's life (although, obviously setbacks will worsen it) and more to do with inertia, for lack of a better term. I think a lot of people on the spectrum are susceptible to this because our brains like things to be predictable and constant. When I had this back in the dark ages before SSRIs, I went to a doctor and got a TCA, and it successfully helped me sort of "reset." I didn't need the meds for very long, either.
All that is to say that I think your suspicions are well-founded: from your description, I am guessing there is something functional that is causing your son's depression. (We have this with my son, too.) The problem is that it's very difficult to pinpoint exactly what is causing them stress, in part because while it may be one single thing, it's more likely to be a combination of many things, each bothering him slightly but together they are overwhelming. I hope you can help your son figure it out. Hugs!