There can be a real value in reading or learning by heart poems that you don't understand, and just relishing the language. Then it's all in your head later on, when you are old enough to fully grasp it.
I also think that Blake's ideas in this series of poems are not too complex, once you get past the language, and if explained clearly 8 year olds (for whom life is quite black and white, isn't it? not so many shades of grey, and still very childlike) will grasp it OK.
He won't get there on his own but if the teaching is good he will get something out of it. Don't worry about trying to 'understand' it - just read it with him. I hope this isn't just homework? Did they work on it in class?