You are not a bad pet owner at all. You have been very responsible and have given him everything you could. I think that if he has more chance of making it through without the x-rays then it would be pointless to get them done. Also, if the vet said there is no treatment with the x-rays then all they would do is put him at risk.
Just love him. Spend time with him and make sure he is happy and comfortable. It is a very hard thing to have hanging over your head. Find comfort in knowing that you gave him a wonderful life and that he is happy with you.