Is your horse still sore? If so, I have a couple ideas of what might be the problem – but first I have a few questions.
How often does he get trimmed?
Is he usually sore just before or soon after he gets trimmed or does his trimming schedule not seem to affect when he becomes sore?
Has anything changed in your riding routine (less time on trails, rockier trails, etc.)?
Could you post a picture of the bottom of his foot when he is sore (the cleaner the better)?
Hoof boots are great for riding, but if your horse is sore there is a problem that you don’t want to cover up by using hoof boots. They are very handy to have around to trail ride a horse that has not had sufficient time to build up callous, but from the sounds of it this is not an issue for your horse.
I hope he’s better by now, but if not I’ll try to help.