The problem with food-snatching is that it's a self-rewarding behavior. That's why it's so difficult to curb. What I would try is to carry super-high value treats -- not hot dogs, not the usual cheese...something that he REALLY wants. For one of my dogs, that's sugar cookies. The other would do anything for cheese popcorn. You get the idea. Use a burrito or croissant if you have to! These are not normal training treats, but something he gets ONLY when he ignores someone else's food and sits politely for you. Sometimes it works really well for my dogs to see me eating a certain treat food, thereby increasing its value. They know the difference between dog treats and my food and will work 10x as hard for a taste of my food! (So nibble that croissant and yum it up right before training!)
You cannot expect him to magically know what to do. You will have to teach him this routine, probably with the help of a friend who can avoid any reaction to going for the food. The direction and reward has to come from you.
It'll go something like this:
Leo sees food, you gently pull body harness lead to steer him toward you, show Leo the uber-yummy treat (use a TEENSY piece so he will want to work for more and so you can give him 2-3 morsels to savor, instead of 1 lump that's gulped down), then ask him to sit -- work on sit separately from this exercise if he needs work on complying with that. When he does, give him the yummy bits, tons of praise, and then release him from his sit and walk on, with tons of happy praise. The idea is to get him to learn that 1) you control all the world's food, and 2) you'll happily give him some IF he's a good dog.
You can't control the idiots at the dog park who bring food in. Most dog park rules stipulate no food, so that's their own fault if Leo swipes it. Unfortunately, success at the dog park means he'll keep trying elsewhere. You're doing the right thing to check this out before releasing him.
What kind of collar/lead are you using? I'd try a harness, the no-pull kind that attaches to the leash in the front, not the silly ones with the leg straps that hobble a dog. You'll get better full-body control over Leo without having to strangle him with a choke collar or yank his neck about with a head halter. If he's really tricky to keep focused, then you can use both a no-pull harness with its own lead and the head halter on a separate lead -- keep one lead in each hand and always, always, use the harness one to direct FIRST, then if you need to keep control of his head, use the other. Keeping one hand close to the snap of the head halter is crucial. The closer you hold the lead, the more control you have and the less pulling you'll need to do in order to redirect him. He may really resist this, so don't only use the harness and head harness (like Gentle Leader) when you're trying to train him -- make them part of his usual gear for hanging around the house supervised and short walks when he's not likely to need anti-theft training.
Food-related issues are pretty common with RRs. Most are sneakier at home with stuff on the counter vs. helping themselves to other people's food out and about, but it is a problem that can be solved, or at least minimized in most situations. My own dogs have taken food out of people's hands -- Finn nibbled half of my husband's sandwich on a hike because Greg wasn't paying attention to that hand while was getting something out of his pack. Lilly has helped herself to other handlers' bait at dog shows (totally my fault on that one -- we were standing there talking and not paying close enough attention to what our dogs were doing).
I hope some of that helps. I'd definitely try some of these strategies before hiring a professional, because if Leo behaves well at home, then this is just a bad habit he's gotten into, not a sign of general disobedience.
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