Very hard to give advice without photos, because there are a few possibilities for what it could be. Normally after a month or so the hooves will have started to callus unless there’s some underlying issue…
The first and most obvious thing to come to mind is disease, check to make sure there’s no thrush or white line disease and the central sulcus is open… no entry wound for abscesses or laid over bars, things like that. Simple diseases are fairly straightforward to treat. An anti-septic, bacterial, fungal combo treatment works well, my go to treatment is a few drops of tea tree oil every few days. Some people prefer to buy over the counter remedies or do something like an apple cider vinegar soak.
Another thing to watch for this time of year is laminitis, I don’t know where you’re located, but if you’re in the midst of spring and your horse is on all this sugary new grass it’s a possibility. Sub-clinical cases are fairly common, and when the horses aren’t lame most owners don’t even notice. Laminitis comes on gradually, I’d check the digital pulse on all four limbs to make sure it’s not strong or really bounding. Feel the hoof and make sure it’s not really hot when the horse hasn’t been working, check for weird growth rings or abnormality like stretched white line, etc…
If the shoe was removed and the hoof was aggressively trimmed, that can cause a tender foot as well that could potentially take a while to grow out and toughen up. Most farriers I’ve met wont trim much beyond rolling the toe a little bit after removing a shoe when the horse is transitioning (even though sometimes it looks godawful) because they’re less likely to be sore.
Just to keep him comfortable for the moment, you may want to consider getting him fitted for boots. Ideally they’d have an insert that would stimulate the frog, but even just regular boots would provide some relief. The advantage of boots is that they aren’t much more expensive than shoes, really, and they’re removable too, which is nice.
I’ve also heard of cutting a homemade pad out of hard styrofoam insulation and duct taping that one to the hoof as a temporary measure, if you decide to try that make sure you use short pieces of duct tape and really reinforce the toe, because it’ll be taking the most abuse.
How is his body condition? If he’s deficient in anything he could be having trouble growing more hoof, in which case I’d suggest adding some kind of ration balancer to his feed or a supplement specifically catered to hoof health, there are plenty of these on the market. If he’s overweight I’d try a supplement such as simmIRdown which was developed for anxiety but also works really well for weight management and as a laminitis preventive… Which now that I think about it I should start feeding that to my overweight escape artist pony. I think the only major downside to this product is that you have to buy it from Canada, but I know of people in the states who have ordered it online.
I hope that helps, transitioning can be so stressful. I had two transitioning here last year and they had ugly feet for the first little while, but once the frog and digital cushion came back into play their hooves started growing out much tougher… Sometimes they just need an extra little bit of help at the beginning. Good luck with it!