((((windblown)))) You are in my thoughts today!! You have gotten great advice already, but just wanted to add my 2 cents from the homeless service perspective. As I have posted here before, I am a social worker. I work in grief and bereavement counseling now, but for years I worked in Homeless Outreach (working with adults with mental illness, substance abuse, HIV and/or chronic homelessness).
No one wants to imagine their child homeless. No one. But the reality is the things you fear --that "Teens on the street are victims of violence, compromising sexual situations that can lead to std's" is not a outcome of homelessness, it is an outcome of addiction. Teens SUFFERING FROM ADDICTION are victims of violence, compromising sexual situations that lead to stds, etc.
Being homeless is obviously never ideal, but there are resources -- there truly are. Many people working in homeless services have a special soft spot for teens and in many areas there are teen-specific shelters (not sure about vegas). The vast majority of people who experience homelessness do not experience chronic homelessness -- they go through it once, for a few weeks or months, get help, and get back on their feet. They vast majority are not victims of violent crimes, compromising sexual situations, etc. If he is committed to recovery he will have resources. Depending on your state, being homeless can actually be helpful in access to certain resources because you get priority. If he continues using, he will still be high-risk for violence, stds, and other issues even if he is under your roof! Take care and stay strong!!