Scabbed, swollen finger?!?

One of my boys, Lupin, has a scab on one of his fingers. I have no idea how it happened, but i noticed it a couple of days ago.

He has bitten the fur from around the area and it seems to have swollen a little more since I first noticed and the scab had gotten more pronounced.... He doesn't seem to be in any pain though. He was flicking the paw around a bit at first but seems quite normal now...

Any ideas what it could be? Should I take him to the vet and is there anything I can do to help him in the meantime?

Thanks!