Is this schizophrenia or dementia or something else?

i know it's schizophrenia or dementia. but i went to school with this kid and he was completely fine. very fit worked out did good in school and was in all different kinds of sports all year long. but then our sopre year of high school he started talking to people who wasn't there, remember things that never happened, didn't remember things that did really happen. He got thrown in a hospital for three and a half months, gained alot of weight within those three and a half months(he was probably 5"2 and 140 lbs, within those months he was 200) and is now really drugged out on a bunch of different pills. but i was just curious what this boy has. it's been five years since then but i had just ran into him and he remembered me and we talked a bit. but then i remember that this had happened to him. and i wanted to look it up a bit. but i'm not sure if it is dementia or schizophrenia or if it could be something else.