Is this common with schizophrenia?

While memory loss and tered brain patterns are a symptom of schizophrenia, it doesn't sound like your cousin has those symptoms exactly. It sounds more like they might be giving him, or gave him, medication that could effect memory loss. With schizophrenia, a person will still remember family and all, but their memory for much anything else is worse. If anything, the added stress of being away from home and confined to a less than desirable atmosphere could also be effecting his memory considering you stated he had a pre-existing issue as far as mental retardation is concerned. I would look more into finding a way to get your cousin better istance where he is, bring him things he's familiar with like pictures from home or maybe a toy/object he likes. It will help ease some of the stress and probably put him more at ease.