Your concerns about your brother are valid. As you suspect, it could be drug use or psychosis (or drug use that led to psychosis). Another possibility, is an organic brain disease but the only way to know with certainty is for him to be evaluated. If you can convince him to go voluntarily, that would be ideal.
He may be telling you the truth, about his not taking drugs, as he knows it to be. He may not remember taking a drug or he may have taken a drug without knowing it.
If he’s unwilling to seek treatment, then consider calling the local mental health crisis team. Many communities have them. You can call them to ask for their advice and to request that they come to your home to evaluate your brother in person. These are exactly the types of situations they are trained for and they can assist you.
You can also consult your local community mental health center, the National Alliance on Mental Illness (NAMI) group (most communities have them) or the Treatment Advocacy Center in Arlington, Virginia, who can be reached at 1–703–294–6001. Here is a link to their website for resources in your state.
In the meantime, do whatever is necessary to keep your brother safe. His rational thinking processes are compromised and he might do something that he would not normally do, as demonstrated by his leaving the house inappropriately dressed. He needs treatment as soon as possible. Psychosis (if that is what the problem is) generally does not get better on its own.
I hope that you and your family are able to find help for your brother. Please consider writing back to let me know how things have turned out. Please take care.
Dr. Kristina Randle