I'd suggest you try things that you would try if you were trying to wean him from some nursings, like offer him a drink, snack, distraction either when he asks or if you can predict when he asks. I'd also suggest telling him that you don't like what he's doing and that he needs to ask you in a way that you approve of. This may take a while, but you can practice at home and tell him that he needs to do it while you're out too.
Good luck, HTH.
The trick that has worked best for us is saying "OK, you can have snacks [our word for bf] for 10 (or 20) and then we're all done." Of course we have to do this on both sides, and now that he can count to 20 if I say "10" he'll counter-offer with 20. But I control how fast I count out loud, so I can make it a quick 20 if I'm really climbing the walls.
Distraction helps, but not always. I'm also trying to be firm and consistent about "snacks all done" if he bites, pinches, or scratches. I will give him a warning - "What happens if you bite?" and he'll say "snacks all done" so I know he understands, then if he does it again I end the nursing session.
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