I agree with getting x-rays. Your horse could have had mild laminitic episodes which have set him back. X-rays are also very useful for the farrier to check and correct angles. In my experience, shoes can easily mask milder laminitis symptoms. I’d also potentially check and/or keep an eye out for IR, if laminiitis has occurred. Any laminitis episodes during the transitioning period will set your horse back, as it essentially flattens the sole.
A balanced nutrition is a huge part of transitioning a horse to barefoot, especially on horses that don’t genetically have much concavity to begin with. My horse, who also has wide and flat hooves (size 5 cavellos), went barefoot after a series of bad shoeing jobs. The first few years were a struggle, as he also had a few mild laminitic episodes that basically set us back another year (time it takes for a new hoof to grow out) in terms of gaining concavity, so we needed to sort out IR. But even on a good record, he was more sensitive to gravel. We only noticed a big change after properly evaluating and altering his diet. This horse now walks on gravel, cement etc with no problems, but this did take time.
I started learning more about nutrition and eventually had our hay tested and factored in high/low nutrients in our area. One thing I’ve learned is that Iron is relatively high in our area and although this doesn’t always appear in hay analysis, horses can get plenty elsewhere, plus every grain in my area has way too much iron in it without enough copper to balance it. Iron deficiency, on average, is rarely caused by nutrition and is more or less an indicator of a health issue. Iron has also been linked to issues with IR. I removed Iron and upped copper and Zinc (1 Cu:4 Zn - ratio), plus altered the other nutrients according to hay analysis and my farrier and I started noticing big differences in my horses hooves around the 6 month period. Additionally, after a year, my horse actually had concavity and his hooves were not wearing down as fast as they normally had. White line connection has been tight since then and we’ve had no issues since (3 years ago). If you don’t know much about nutrition, I always advocate for reading about it in sections, but I think the easiest/ fastest way you can balance your horse’s nutrition is through hay analysis and either having a equine nutritionist unaffiliated with feed companies to evaluate the diet or send the hay analysis to a company such as Horsetech or Mad Barn to formulate a custom supplement for you. It may be expensive at first, but you can look at the ingredients and amounts of vitamin/ minerals and find cheaper and similar alternatives down the road. The latter route does involve calculations on your part, but it is very rewarding to be able to have that knowledge in the long-run.
Also, do take into consideration the surfaces/ level of work your horse is doing. If he is riding on hard or very rocky surfaces for longer periods of time, then he will need something to help protect his soles, whether that be shoes or hoof boots. I’ve used the cavello simple hoof boots for my horse, although I know there are certainly more trail friendly designs out there. Personally, I was a bit limited as cavello was one of the only, if not few boots that would fit my horse’s hoof properly. I do know someone who uses scoot boots though and they would probably be my choice if not for the limitations. I see durasole has already been mentioned and that is also great to try. Another one that I had frequently used and built up for some sole protection is Venice turpentine. My farrier had also used this: https://hoofarmor.com/hoof-armor-order/
My current gelding, the production of that stallion and one of those two mares, needs shoes for most work we do or he wears too badly. My wife’s mare can do easy stuff around the farm but does not do well on hard surfaces.