When I was in my early twenties, I was diagnosed with sz. One reason was because I was “responding to internal stimulii.” But this was not debilitating for me and I lived just fine with hallucinations for many years. Only sometimes did they become overwhelming. About six percent (if I am remembering this number correctly) of people hear voices. Only in certain illnesses and at the worst times do the voices become debilitating symptoms. Some people who are managing sz quite well hear voices often. It’s not necessarily the end of the world if a person is hearing voices as long as they are doing well in other ways. The problems come when people are unable to access shared reality and start living in the alternate realities of psychosis. If someone is connecting and connected to reality, they might be able to manage hearing voices without stress or debility. In sz it’s hardest because of all the other symptoms and difficulties that occur.
My advice is to look for ordinary behavior. Interact and relate as ordinarily as possible and don’t worry about this unless there are signs of debilitating illness. If you think your son is doing better, he probably is.