Mr Woof is not a collie, but he does have that fixation thing - so much training advice centres around using treats or “high value objects” which don’t have any impact because once he is fixated there is absolutely nothing else of interest to him. If he eats a treat in that state it is only because it is the most expedient way of getting it out of the way! All the advice I have received about how to deal with that is to start further away from whatever he is focussing on when he is still distractable so that you can work on it gradually, but he is like an on/off switch, there is no in between to work with.