Mental illness creates huge barriers when trying to help folks out.
Folks with mental illness do not want to admit they have a problem and or do not believe they have a problem. Some may, at the insistence of their family, get temporarily admitted for diagnosis and treatment but most stop taking their meds just weeks out of the hospital.
Normally a city has some social workers ... etc... who monitor the homeless population; also wise to check with the police force to see if they are aware of this VETS dilemma and location. They may know him well but ignore him since he stays way below their crazy radar.
Mental illness is the world of the Psychiatrists and psychologists but it often ends up in a social workers lap just do to the pass the problem attitude that exists in the U.S. since we emptied out the mental institutions starting back in the late 60's.
I tried to help a couple of these homeless folks / Vets out and it bit me and I learned that my job was to talk, encourage and offer support but to avoid becoming the thing they were dependent on - that can ruin your life. Good Luck.
Have you learned who his family is and where they live?