Eat at anything

I have a 7mths JRT. He is very strong and hyper. Everytime I bring him out for a walk, he goes into hunt mode. i.e. He will chase at every cats in sight, sniff around the grass fields, eat at any junk food or even cat/dog shit. I'm very afraid to see him chewing something as my friend's dog had died from eating garbage food on the streets.

Hence, Is there any way to prevent him from eating outside. How to take those stuffs from his mouth without hurting my fingers. I tried to feed him half-hour before the walk, but it doesn't work out.

Please advise.