As an adult who needs medical help and has no insur I may be qualified to answer this.
Medicaid varies by state so you'll have to check your state. But very few, if any, would put him on Medicaid. And that may not be the best answer anyway. Once he's on Medicaid the doctors he can use may be limited. medicaid may restrict the medications he can get. And FYI most states have now signed up Medicaid with an HMO.
You need to check the local public hospital for a sliding scale program. That is, the fee will be based on his income. No question it's a lot of paperwork, but signing up for SSI isn't easy either. Once he's with such a clinic they should be able to get whatever kind of care he needs. You should also check for a mental health nonprofit near you. They will only do mental health but they may have groups that would be helpful to him that the hospital doesn't have. This kind of place, in my personal experience, is familiar with the attitude and behaviors he'll exhibit in a way the hospital clinic isn't. That is, they'll be more tolerant of missed appointments, able to give him samples of psychiatric medications, etc.
In terms of medications check needymeds.com. They list all the pharma programs that will give him free medications once he has the script from a doctor. Again, some paperwork involved but it will give him free or cheap medications.
My suggestion would be that you work with him to get all his paperwork organized. They'll want W-2s, tax returns, etc. Would be helpful if he's had a series of jobs to have them all listed with time frames.
To find programs:
for psychiatric programs call the hospital psychiatric ward, local suicide hotline, local ER,.... ask who they refer people who have no insurance/money to.
for hospitals with sliding scale clinic go to the medicaid office and ask them. note that for one pharma co medication program I needed a turn down from the medicaid office in writing.
for "public health issues" eg hepatitis, flu, AIDS, STD, TB... call the local health dept and ask who they refer to for free tests/ medications if they don't do them.