I've been keeping an eye on it. Occasionally it has a bit of heat, but it still doesn't seem to hurt him when I play with it. He doesn't limp (any more than usual xD He has a habit of limping when we get to the top of the track we ride on, right by where we usually stop. He puts up a fight if I try to keep him going, but one we do the limp miraculously disappears, until we get to that spot again!), still runs around the paddock like a mad thing, so I'm not really worried, but I am curious.
I realized the other day that the position is at the right height for the corner of the old safe I renovated into a feeder - is it possible that repeated bumping of his knee on the corner could cause the lump?