Let me share a story.
An accomplished masters' runner asked if I would coach him for the National Senior Games. I wondered if I could help him since he was already very good... and because he was recovering from cancer surgery. We did a track workout together and I immediately noticed two form flaws. I pointed them out and he corrected them over time and improved. After he fixed these two problems, I became aware of another problem. It was costing him probably a full second in the 200m and all in the finishing stretch.
I explained what he was doing wrong. I gave him verbal mantras. I shouted cues as he ran. Nothing worked. In desperation, I tried a different cue. Another different cue. Another. And then, finally, one cue clicked in his brain and he did it right!
When he finished that rep, I said, "Okay, this your cue. When you get 70m to the finish, I want you to tell yourself this cue." He did. It worked. He finished 2nd in the 200m and ran an age group PB.
The cue wasn't obvious. It wasn't the top ten of things I personally would think to say to myself, but it worked for him.
My point?
Maybe your runner needs a cue. A mantra. Something to tell himself as he is running. Maybe at various stages of the race. I don't know what those cues are, but you might think of some and give it a try.
Maybe some posters could share what works for them.