Is he sore all over or just one foot? If just one, it is likely there is a bruise or abscess and I would ask your trimmer to take a closer look at it.
And I agree boots are always nice if you are riding on particularly rocky areas. Have you been on these trails before?
You could also look into something like Durasole or Keratex hoof hardened to help toughen his soles.
But either way, sounds like a good discussion to have with your trimmer
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