As you are already about to do check foot for thorns stones cuts etc. We find it best to get somebody to do tummy rubs whilst the other looks at the injury.
Blackthorn is our biggest cause of foot probs but a sharp stone or old tile in the ground can also cause nasty cuts/abrasions.
Once you've got it clean if theres no obvious cut/abrasion, chances are he's just twisted something racing around,if theres a cut i would spray with a Terramycin spray (from vets) or purple spray if thats what you've got. We have also found horse 'hoof and heel' quite good for sealing over cuts and keeping mud out as its paraffin based.
HTH
Mandy