Dear friend,
The fact that it has not grown spread or worsened may indicate is it not malignant but a more simple bone spur or tumor.Here is more info on osteosarcoma:
http://en.wikipedia.org/wiki/Osteosarcoma
I understand what you are saying as far as breaking his spirit because he is trained to perform. If that ability is taken from him he may become depressed and suffer mentally.
Take to the oncologist about interferon which can boost the immune system. I would also consider the homeopathic options on the above site.
There are many non malignant bone tumors, cysts, polys, etc that could be causing this. Howebver, you should have a yearly biopsy to make certain the growth is not changing so at least you will know where you stand. You should also consult an orthopedic specilist that is experienced in bone growths in the labrador retriever.
I will also consult with other experts so you will get the best available help.
I will be praying for your boy.
With utmost sincerity,
Terri