My Brother Suffers From Slurred Speech. This Doesn’t Happen All

My brother suffers from slurred speech. This doesn’t happen all the time. He can go days and be fine. It normally happens in the evening. He is 40 years old and generally in good health although he suffers from severe depression. He has also recently admitted to having an alcohol problem (we have known about this for some time). He does not drink all day everyday but when he does drink he does not know his limits and things normally end badly. Because of this, when his speech starts to slur, we automatically assume he has been drinking. Last night my mum drove to his house in the middle of the night to find him fast asleep in bed, the house safely locked, he had been cutting intricate patterns out of leaves with a craft knife (so couldn’t have been drunk). Also, there were no alcohol empties anywhere only root beer (non alcoholic) cans. What could be causing this slurred speech? Diet? Blood sugar?