What does Willy mean when he says "You can't eat the orange, and throw the peel away - a man is not a piece of fruit. " to Howard?

"A man
is not a piece of fruit" is a direct rebuke to consumption. The company where Willy Loman
worked as a salesman, first for Howard's father and then for Howard, is discarding him, as one
would discard an orange peel. Like an orange peel, they have no use for him in his elderly,
senile state. 

They have taken, or consumed the best of himthe
"fruit" of his youth, as a previous educator has mentionedand they are now throwing
out what they find distasteful about him: his age and slowness. 

Indeed,
theis uneven and somewhat nonsensical, but I think that Miller intended for it to be. Loman is
losing his mind, which would make him unable to form good analogies. Howard has also reduced him
to a feeling of ineptitude. His language demonstrates that.