Any idea what this is? It’s causing pain to my dog and he is limping

Hello there - limping is indeed a manifestation of pain; we limp because it hurts to walk normally (except for rare cases where, for instance, the nerve doesn't sent the right motor signals to the foot).  In this case, there looks to be a lumpy structure under the pad, which would likely cause pain when the foot goes down and thereby a limp.  Such a thing could be a cyst, a foreign body / reaction to a foreign body, a tumour, bengin overgrowth of normal tissue or a lesion associated with a systemic illness affecting the growth or quality of the pad.  These can include leishmania (infection), distemper (a viral infection), pemphigus (autoimmune disease) or zinc deficiency.  I am no expert as to which avenue to persue here, but your vet could find out more by looking at the lesion in context of the rest of patient / clinical signs etc and perhaps in coversation with a pathologist and / or biopsy if it is not obvious to them.  Hyperkeratosis in the cocker Spaniel, for example, is often treated initially by trying to soften the keratin and addressing any uneven or inappropriate weight-bearing. I hope that something here is useful.  

I have returned as I realised that I wrote all of this without mentioning the word 'corn.'  A corn is hard area of keratin overgrowth, which can occur for multiple reasons, including irregular weight-bearing or indeed any of the other reasons mentioned.  Whatever the diagnosis, a vet has two roles in this matter;  the first is to address any pain associated with the lesion being under the foot.  They may also be able to reduce the symptoms.  The second is to consider tests to identify (and hopefully to treat!) the underlying cause.  Please do let us know how you get on.

Thank you so much for the thorough analysis! A vet has taken the skin sample for testing. I hope to at least find out what it actually is and use the right way to cure it. 🙏🏻🙏🏻🙏🏻