Hi XXXXXXX~
Because there's an actual lump involved...which could be many things from an underlying fracture to a foreign body...I think the best thing to do is to have your vet take a peek...especially if keeping him off of it hasn't helped.
My guess is that they'll probably take a quick x-ray to rule out fractures, and then will probably try him on an anti-inflammatory/anti-pain medication to see if that helps bring the swelling down and make him more comfortable.
I hope this helps.
Certified Veterinary Technician
Associates Degree in Veterinary Technology. State and Nationally certified.