How do I get guardianship over my 29-year-old son?

I would get a legal opinion from an attorney in your jurisdiction. Laws vary by state and I'd see about your case, based on your facts. I know it must be so frustrating to deal with this kind of struggle. I have seen friends go through this

It is very difficult.

If your objective is to make him take his meds, guardianship does not mean he will take them.

I believe you have to get 2 doctors to certify that he presents a harm to himself or others.

It’s so frustrating! I know from personal experience what you are going through. I did not pursue it as my family member doesn’t meet the criteria. He can cook, drive, pay his bills, hygiene is no problem, but his has a mental illness and refuses to take meds too.
I just monitor him and keep in touch with him almost daily.
I am sorry you have to cope with this. It’s a real heartbreaker of a situation.

You get an attorney. You will need to provide backup documentation that he is a danger to himself - from a qualified legal professional such as a medical doctor and/or a psychiatrist. This will prove difficult given privacy laws as they relate to health care. But once court proceeding begin that will be required information.

Be warned that if your son contests guardianship this will likely become a lengthy and expensive process.