How do you treat a sick guinea pig (tumor, cortisone)?

Hello everyone, a tumor (testicular tumor) was found in my guinea pig in June. Due to his advanced age (8) he was not operated on and was totally fit!
Then about a week ago, one of his hind feet started not obeying as well and was sometimes dragged (he can feel something in that foot though) and as a result he can no longer walk properly and sometimes can't stand either. Since I always work from home, I can supervise him and he is always with me. I took him to the vet again and she gave him a cortisone injection (once and quite a small amount, but animals don't weigh much). Then he was much better for two days (he could stand for hours and even walk a little without my support). But now he's basically back to how it was before.

I don't know what to do with him. He's otherwise in a great mood and eats a lot (according to my vet, everything's fine as long as he eats) and has a total will to live. Do you have any experience with cortisone treatment? Has it been done often? Having to go to the vet every week for a cortisone injection might also be a pain? Although the vet visits really don't stress him out…

I ask you for factual answers, and not to be too harsh, as it is a very emotional topic. I would simply be interested to know what others have done in a similar situation and how the treatment with cortisone or other means went.

Thank you !