Seamus and I have been working hard on leading - he didn't really have a 'forward'

and I've been working on it by waiting when he stopped and not getting after him by swinging ropes or
putting any pressure on him at all, this was great when we were alone in our isolation field
all summer, as we didn't actually have to go anywhere!! Now we are back on the yard and in and out of the stable/field
he is happy to go and very rarely plants

When he does plant it's because he's scared of something ( never sure what

) so I did 'head down' to calm him
and then we went forwards again, the trouble is that 'head down' then meant it was ok to eat grass.
So I thought...... OK...... training hole identified
I needed an 'eat' and a 'dont eat' cue.
So eat was 'stroke mane' and 'head up' is vibrating lead rope and making kissing sound - mega favourite treat of course for
actually getting head back up again.
It's all hopeless though as now he just decides to eat grass when I'm taking him to and from his field it takes ages to get him
to leave it and that's only when he's got his mouth so full he cant actually eat any more. I do click and treat him when he does put
his head up but feel aggrieved as he wasn't asked to eat and he didn't leave it in an acceptable time but I'm not sure what else to do ???
In the olden pre-clicker days he didn't dare eat

so I've made matters lots worse

when in general our relationship has got better so I don't want to go back to dragging him around and lots of neg reinforcement.
Can anyone give me some direction on this one?