Eats whatever he finds outside HELP

Hello,

 

my 1.5 year old Papeagle Charlie, when we are on our daily walk, eats whatever he finds outside. I tell him to leave it or drop it, the commands he knows well, it doesn't work when we are outside (on leash for sure). As soon as he realizes i see him eating he chews and swallows so fast, he knows exactly it's wrong. He finds everything from chicken bones people threw in the grass or bushes to not so nice things. I am very worried one day he finds maybe rat poison and dies. Or some other toxic stuff. He is not doing that because he is hungry. I even started to "screen" the streets and grass before he walks there. But still, he always finds something. He is a Papillon-Beagle-Mix and acts like a hound, nose on the ground and tail up in the air constantly, since Beagles are scent-dogs no wonder he finds all the yummy stuff