Hello, my name is***** and I have over 20 years of experience as a veterinarian. I am sorry to hear that Sam has a big knot on his right front leg, is not using that leg normally, not eating well and is panting a lot.
I think he is panting a lot and not eating because he is in pain.
If he doesn't have a fever then this is less likely to be an infection, but it is still possible that this is a a fungal or bacterial infection. Those are treatable.
At his age however cancer is probably the most likely cause of his symptoms. If the mass is very firm, attached to bone and not moveable then a bone tumor is possible, and those are very painful. They also tend to be very aggressive. If this is a bone tumor they can metastasize (spread) to the lungs, and that could explain his panting as well. Where is the mass located on his leg? Is it firm and attached to bone, not freely moveable?
While you may not choose aggressive treatment for an older fellow if this is a malignant tumor there are medications that can help with his pain and give him a better quality of life for a period of time. I know you don't want to see your old friend suffering and palliative care to control pain and then euthanasia when we can no longer control his pain is a good option.
But first we need to know that this lump is, and that means a veterinary visit, radiographs of the affected area and possible aspirates or a biopsy of the mass.
In the meantime keep him quiet. The more activity he does the more uncomfortable he will be, and if this is a bone tumor we can see the leg fracture as the tumor weakens the bone.
Please let me know if you have any further questions.