I shared horses for years and would never put new bedding in unless I was asked to do so. I ended up being really good friends with all owners and even then, I wouldn’t assume it was ok to put fresh in (I would ask if the bed looked thin). I was also really open to get guidance on mucking out...
What a rollercoaster you are on at the moment. He is fighting so hard for you, I so hope you have the miracle that we are all hoping for. Your vet sounds amazing. I trust my vet explicitly and trust him to tell me when enough is enough. I have everything crossed for you both. xx