swelling on back of pastern, any ideas?

[Billie]

Billy has had a swelling at the back of his pastern for about 8 weeks. I am going to call the vet out now for it but I was wondering if anyone might know what it is.

It is not a firm swelling and you can push it right in. He also has some warmth around his coronet band. He is not lame.
Any ideas?