My mare got something like this earlier in the year. The lump was hard though then broke open & got infected :/ turned out to be a ground bee sting. Darn thing is FINALLY going away. Her's was a little taller than what your boys lump is, but ya never know... hopefully your vet does! Posted via Mobile Device
So, the vet also has no idea what it is. He said that it's possible that the tip of one of his spinous process (vertebra) may have fractured off, and that we could do an x-ray, but probably wouldn't be able to do anything about it. So, I'm going to go with the best guess and say it's a piece of bone!