I think bringing him inside was a good idea. Maybe make him an indoor space where you are at a lot so he can see you all the time. You might not have time to actually play, but he will probably feel better if he’s around you. My rabbits go CRAZY if they can’t be with me for more than two hours when they are awake (they sleep while I work). SO, I think you are doing great so far, and I don’t think you really need another rabbit to fill the spot where the other one was. Just fill that spot with your own love
Rabbits grieve just like humans, and it’s a process. It will probably take awhile.
Also, if you suspect he may get sick, I would keep him indoors to keep an eye on him. That is one of the benefits of being a house rabbit, rather than an outdoor rabbit. You can see signs quicker, and rabbits don’t take long to go down hill. I really hope Benji starts to feel better, and I think you are on the right track!