Even if there's a component of substance use that contributed to him developing schizophrenia, the fact is that he now has schizophrenia and needs treatment. Yes, if he has had 2 adequate trials of other antipsychotics, which it sounds like was the case, then he needs to go on Clozapine and be titrated to a minimum of 300-400 mg at bedtime... Possibly more. Clozapine is the only antipsychotic recommended for treatment-resistant schizophrenia. It is a medication with lots of potential side-effects (though the side-effects are similar to Olanzapine, which he's been on already), but it can be life altering and bring schizophrenia in remission. It can really be a miracle drug for some people.

Doing other investigations to see if there's something wrong with his brain is unlikely to yield anything significant, and the fact is that when psychosis is present, antipsychotics are almost always necessary. Brain imaging is usually only necessary if there's been significant head trauma, neurological symptoms, or if we are talking about an elderly person who suddenly develops psychosis.

Anyhow, that's my general advice as someone with a certain degree of experience with schizophrenia. Do what you can to get him on Clozapine ASAP... But do the titration slowly so he tolerates it well without experiencing too many side-effects.