Branston (age 8 years) wears a basketville muzzle when he is out - he too picks up anything he can without it on - as well as blockages I am concerned he will pick up something that will poison him . Altho I know he is not keen, it means less of a stressful walk for both of us, as he can run around with it on.
He is the golden one that you can see in my avatar, and he wears a No. 6 muzzle, but for best fit if possible take your little one to a pet shop/outlet and try one on for size (maybe a size 3 to start with depending on the length and breadth of his nose. It needs to fit snuggly but not tight.
If you look in the Behaviour and Training section - there is a thread started by Andy B entitled 'scavaging' where muzzles are discussed as well.