Barley fever is slight sexual arousal due to intoxication.
Court Road is a road in London, lots of trading, selling etc.
Monkey on his shoulder is a curse, a problem or, in this case, more likely something that obsesses him.
Monkey getting "Grinner", ... - Grinner is one with a perverted or sexual sense of humor.
So the obsession is becoming perverted and or sexually humorous.
Eyes are on the ground..... He/she is actively observing the situation....opposite meaning (like ear to the ground)