If they're soft there is no real rush to get him into the vet. He could have just bruised himself and it will go down on it's own in a few days.

If it's still there in a week or so or it continues to enlarge in the next couple of days, then he should be seen by the vet.

Most fatty tumors are benign, but even If it does turn out to be a fatty tumor the vet will want to remove it and biopsy it just to be on the safe side.