Definately a hard one but you can try training the „leave“ command but outside with a lead and using different things ranging from treats to toys etc.
Humphrey (4) still always had his nose on the ground and a bit of fresh cow poo before breakfast
is his idea of the ideal start to the day
but on the whole, I found that the obsession of picking up everything single thing did wear off as he got older.
You can only really just keep on training it continually (lots of good videos online), hope that it does improve as he gets older and, as a very last resort, consider a muzzle. Cockers are ruled by their noses and scaveging is unfortunately something that a lot of them, (although not all) are prone to do!
Best of luck!