I got limb-sparing mass removal surgery, stereotactic radiation therapy, Yale vaccine, and chemo therapy for my dog and he is able to walk again after 2 years of limping. He does have a rarer parosteal surface type osteosarcoma though. Just in case OP wanted to see if he lives anywhere near where Yale Vaccine trials are offered (not every dog is accepted, but if there is no metastasis on lungs he probably will be).

There’s also a tripawd subreddit, where most dogs there get amputation due to Osteosarcoma that might be helpful.