To add to Wb’s comment, you can practice this at home....he comes to you, has his lead put on and gets a treat, then you immediately take the lead off. Same with being fed, lead on, feed, take lead off. Don’t try and lead him around, talk to him, apart fro m praise, or anything. Just call him, lead on treat or feed lead off. The treats you give should be his absolute favourite thing and NOT given for anything else. While he is learning this, then flexi while he is out at walks. Once he is doing this at home with no hesitation and WHILE YOU HAVE THE FLEXI ON, do it while you are out walking him....same thing, call, lead on, treat and lead off. If he doesn’t respond, resist the urge to haul him in on the flexi, just put the treat away and carry on, try again a minute or two later. Don’t try it when you think he is unlikely to come to you. This does work with (most ha ha) dogs if you persevere because you are replacing the lead on in the park=go home with lead on means treat. The way for this to work though is that lead on=treat has to way outnumber lead on = leave park.