You have no way of knowing if his depression is not related to neurological problems. http://www.ncbi.nlm.nih.gov/pubmed/16612216
He did say he had another diagnosis. I didn't say there was a neurological condition called "depression". You should be more careful when you read and when you reply, and perhaps try not to get so flustered over terms.
Perhaps it would be better to refer to it as "biological", that would cover clinical depression and a wider range of disorders.