First- realize it's not on you. I'm willing to bet you're doing everything you can as a parent. MI doesn't chose based on who raised who.
Second, there's a lot of sound advice here. Calling a doc is pretty important. ER is POSSIBLY importanter. Depending on the severity of the situation Baker acting (or 5150 depending on what they call it in Texas... you're in Texas, right? ) can be a useful tool, but it's berkeleyin' scary and not at all pleasant for anyone involved- it's definitely one of those "for the greater good" scenarios.
I think the MOST important thing you can do is reach out yourself. It's entirely possible he won't say anything about the situation. I know when I was that age I could have actually been on fire and I wouldn't have told anyone. HOWEVER, I can also tell you that when you're feeling your absolute worst, sometimes what you need is someone to reach out. The trick is A- to make it not seem like you know what's up, and B- DEFINITELY do not out your source. Trust is a major player here, and you don't want to ruin that.
I would definitely approach him though, and make a greater effort than a "Hey man, you ok?" situation. Ask more in depth questions. Lead in if you have to. "I noticed you've been (insert concerning behavior here) and it's got me worried. I'm trying to let you do you, but at the same time, you're my #1 concern. What's going on?" That sort of thing. You might get nothing. You might get something. You might get a full breakdown. At that point it is CRITICAL to be as supportive as you can without being judgemental. No matter what happens from that point forward, your only concern is making sure he gets the help he needs. You might hear some E36 M3 you're not trying to hear. Probably a lot of it if I'm honest. Whatever happens, its gotta be let go right then and there.
The next thing you probably need to think about is something called a safety plan. This is a "what do we do in the event of" plan. We know who to call if we cut an arm off, right? Mental health isn't one of those things people are generally prepared for until E36 M3 goes REAL REAL wrong. The idea here is to have a plan(s) in place so if it does go sideways, protocols are set aside. That's where the professionals come in. Who's the doc. Where is the ER. What immediate resources are available. Assuming you move forward with MH care, what sort of plans are going to be in place for handling of medications, etc. Same as you would for cancer or whatever, but with your thoughts, if that makes sense.
Beyond that???? It all depends on how it shakes out from the beginning. I think the VERY first question I would ask, and probably not to him is "did he have a plan". When I referred people out for a living the VERY first question we asked was this an urgent or emergent situation? Emergent meant that they had a plan and things were fixing to go down. This was a 911. Urgent meant...closer to what I'm hearing here. Mention of SI is in place, etc. We're not waiting 30 days or whatever our protocol was for referrals. If he's making mention of self harm, he's asking for help, in a way. One of the REALLY morbid things about this that people don't often talk about is, when someone has made up their mind, really truly made up their mind- they don't usually tell anyone. They've made their peace. They've punched their ticket. The end is in sight and all they gotta do is get there. You have that on your side.
IN ANY EVENT- know that I am not a professional. I've just done life on hard mode for a long berkeleyin' time and had a lot of practice. If you need anything, LMK.