Down to one rat

So, In a previous post I asked for help regarding my elderly rat who had a large tumor growing on his side, unfortunately the vet did decide the best course of action would be to put him down given the location and size of the mass, as well as his age. Now I have a problem, I am down to one rat. He is behaving normally and I have been spending as much time with him as possible, but with my job I can not give him the attention he needs. I know everyone says to get more rats, but this isn't an option for me. Given that he is older I was hoping to find some recommendations on what to do. As I said before, he is behaving normally (eating, drinking, playing) should I give him away to someone who can provide him with the companionship he needs? Or would it be ethical to let him live out the rest of his days solo with me? I would really appreciate anyone with experience on this weighing in.