How is a person seeing a psychologist different from seeing a psychiatrist?

No difference. A patient seeing a psychologist or psychiatrist are not different. It is the professional who treats them who are. Psychiatrists are fully trained medical doctors who after graduating medical school enter a 4 year medical program where emotional illness and treatment is emphasized. Psychologists are not medical doctors. There are many types, many with post graduate degrees and highly skilled.
He may need both. Psychiatrists are md's with specialized training in diagnosis & management of mental illnesses..They are knowledgeable in the use of psychopharmacological and therapeutic treatments of psychiatric disorders. Psychologists are ph.D's trained to perform standardized tests (e.g., iq, social understanding), diagnose & treat mental illnesses using a variety of evidence-based therapies.
See below... Psychologists provide psychological assessment & psychotherapy. Psychiatrists provide psychopharmacological interventions (medication). Some do therapy as well. Many people in therapy may not need meds & may not have a diagnosis! in this manner, therapy is sought out by patients with a psychiatric diagnosis, but also by folks who are just wanting to grow, function optimally & live more fully...