Not saying the guy has schizophrenia, but he was in his late teens when this change occurred. This disease commonly begins in the very early 20's, but it can manifest a little earlier. Some drugs, such as marijuana, can alter brain chemistry too.
Some might call him a highly functional paranoid schizophrenic. He probably has OCD also. Psychotropic drugs and obsessive eating beliefs not helpful in terms of nutrition and toxic compounds in his brain.
How did he find out about the people in the house howver?