Just quietly, his fitness will be less of an issue imo if he plays back. His competitive streak seems to feed more into being a defender than a forward; he has to create play and space when playing in attack which I think he struggles with when he starts to feel tired.
But when he's in defence, he's reacting to the ball, he hunts for it in the air and doesn't want to get beaten so he seems to run games out better.
Good luck to him. I hope it works out and we just let him find a spot in defence at some point because he's genuinely ******* insanely talented and I think he needs to feel like he belongs for a while as well as get fitter before we even consider throwing him in the middle/up forward again.