Dog with prostate problem, please help? ?

My nearly 6year old dobermann has had ongoing prostate problems for 2years. He is treated with 2medications to reduce the size and to treat possible infection. Our intention is to have him neutered. Two weeks ago he went to the vets with the symptoms of enlarged prostate, he was again given 2medications and an injection for pain and anti inflameation relief. Within 2days he was back to his usual self, eating better and more active, he was to be cartsated 2to 3 weeks after the treatment. About 1week after he had been to the vet he began having stiffness in his rear end and one leg seemed painful. He had problems getting out of his bed and walking. Again straight to vets, who said the prostate was enlarged but less so than the previous week. He was given pain relief and antibiotics to treat a possible infection. I was concerned it could be cancer, but, the vet said it's unlikely and to wait until he finishes his treatment. She said if he doesn't improve over the 5days to bring him back. Today he seems to have better movement and is toileting well and no blood in urine. I am still worried it could be cancer which has spread, but if it were would antibiotics (noroclav 500mg) and metacam mask cancer symptoms? I'm worried but don't want to over react as it's only 2days since starting treatment and I notice improvement. Any help is appreciated.