You can get your dad help but it won't be easy on either of you. In the United States, you have a legal right to petition for an inpatient mental health evaluation. I advise that as a last resort, though, and it would go much more smoothly if you could talk your dad into visiting a professional on his own to discuss his feelings.
If you feel he's at immediate risk of harming himself or someone else, and he won't seek help on his own, you can contact your local mental health crisis center to find out the procedure for having him evaluated. It is important to call them FIRST unless there's IMMEDIATE risk. Law enforcement professionals are not the most appropriate resource unless no one else is available.
Most places in the US will have specialized crisis teams of psychologists, counselors, and social workers who can either meet your dad in his home or talk to him on the phone to assess his safety. If they determine he could benefit from further treatment, they can recommend inpatient or outpatient treatment. Depending on his financial resources and insurance situation, that might be a public or private facility.
Also, please know you can call the crisis hotlines as well. You can call the suicide hotline - national or local - tell them your situation and they can talk you through the next best steps for your situation. There are many resources online that offer signs that might indicate impending self-harm. I used to hand out cards in my office with warning signs.
Some things to look for, although this is NOT something that can be predicted even by professionals, is that the individual might start giving away his belongings, saying good bye, being overly withdrawn or, if that's his normal character, being overly engaged. Basically, if something feels really off, you have to trust your gut here.
Having this conversation won't be without consequences. A lot of people are deeply offended/embarrassed/angered that someone would question their mental stability. There's so much stigma attached to mental health diagnoses and self-harm that being seen as weak might even push someone into a more symptomatic experience. As an example, I used to work with individuals who were fresh from ICU after a nearly completed suicide attempt. So many of them were angry that they lived because now, their problems were compounded by medical bills, public knowledge that they tried to self-harm, and mandatory treatment.
The way you are feeling is natural. This shit is scary. I know professionally and, more importantly, personally. My mother attempted suicide many times and nearly completed at least twice. I called the sheriff to help her many, many times (crisis teams weren't as available back then... crisis team should be your go to now). It never got easier and she was always super fucking pissed... but I did it as recently as this past year when she was making threats.
You're a child, regardless of your age, and being flipped and now in the position of caring for a parent is fucking twisted and hard. It might be one of the most difficult things you ever do but I'm in your corner wishing you the best of luck. I'm sending good wishes to your dad too. He sounds like a resilient man... I hope he can find a way to come out of this mess on top. Hopefully, most of this can be resolved with a conversation between the two of you and, if not, please remember there are many professionals who deal with this every single day. You are not alone.