How To Confirm Schizophrenia?

Hello , I would be really pleased if you could possibly tell me what the hell is wrong with my cousin ? He is 26, and he always seemed to not be really smart, but he worked, drived, talked, laughed ... but still as a kid like he could play a game for hours and he didnt get bored of it, he would laugh more than others in a funny thing, and he had difficulties in learning (but he writes and reads, and does some math perfectly), but he seemed pretty normal , and you could talk to him , and he could even give me advice sometimes. Three years from now, age 23 he really lost his mind , he started talking like a mad person, unrelated things, he started behaving really weird, and really really aggressive , i was scared to be alone with him because he talked like a mad person, and weird. From time to time, he is okay, but then again , he stops working , doesnt brush his teeth, doesnt even eat , he just smokes cigaretes all the time, like 3 packages a day , and drinks coffee all the time. His hands shake a lot, and when you talk to him , he responds after a while .Sometimes he is back again to normal (but there are still signs), the other day he looks grose . I feel realy bad for him , and i dont know what the hell can be wrong with him , do you have any idea ... thanks a lot doctor:) , God bless you ... he was always kind of lost, like not really smart, but it was normal to be like that .... PS. he also had a majbor brake up with his girlfriend three yrs ago, and she broke up cuz he got ill .... and so he is going down day by day .... doctors never gave a right diagnosis, some said schizofrenia, some said its just depression , some said its stress .... what do you think ?? thank you a lot ...