I’d suggest glue ons (the easycare octos go on with only superglue, although they do require you to have the right size) or booting while he’s uncomfortable if you want to keep trying for barefoot. It’s worth seeing if he’s comfy in the stonedust lot, but I think some sort of protection would be worthwhile while he transitions. Unless a horse is really remarkably comfy, I do prefer to use boots pretty liberally during the transition.
On the flip side, while I am a huge advocate for barefoot, and failing that, boots or composite shoes, I don’t think it would be a terrible thing to keep an old retiree in regular steel shoes if that’s what he says is comfy. Hoof rehab is important and rewarding, but you also do really need to weigh quality of life and how much improvement you can expect when they’re getting up there in age.