Any idea what this is on paw pad?

[fairgame84]

He's had it a long time but it's started hurting him now and he's limping.
He flinches when I touch it.

We went to the vet about the limping a few weeks ago and she said its arthritis but medication hasn't helped and we think it might be this now.

Any ideas?

I am no medic so do go back to the vet but could it be a corn? What breed is you dog?

He's a mongrel. Looks similar to a golden retriever.

This is him.

There's a really good Facebook page, pet vet corner.
Vets reply to owners questions on there.

Looks like a corn, they can be very painful, needs cutting out by a vet. Definitely get it checked again by the vet.

Thanks @3dworld I've requested to join.

Definitely looks like a corn after a quick look on Google images.
I'll call the vet tomorrow.