Grieving feral teenager - is there any way I can help him?

I volunteer with a Texas-based cat rescue that often assists with TNR. 

About three weeks ago, I helped a woman trap two teenage semi-ferals. Both were male littermates and about 4-5 months old.  I brought them to my home and kept them in a condo until their neuter appointment.

Both got through the surgery fine, but one of the two brothers suddenly became very ill with panleukemia (distemper). I took him to the vet but we lost him later the same day.

In the meantime, the woman I had assisted with trapping blamed me for the kitten's illness. She decided not to let me bring back the other one to rejoin his colony.

Here is my problem: the kitten at my home is grieving over the loss of his brother. He is not eating, and although he is not 100% feral, he stays under the sofa most of the time. Sometimes he will come out, walk around for a few minutes, and return to his hiding place. 

I believe that the best solution is to return him to a familiar place - his original colony - but since the woman who I helped with trapping won't take him back, I'm seriously worried about him. Today I returned him to a condo, made him a bed out of a box so he can hide, and given him a dose of mirtazapine per my vet's suggestion. I am also brushing him, although it doesn't seem to have any effect. He will not touch any sort of food or water I offer him, even the reliable stinky, fishy foods.

Is there anything else I can do to help him recover? It's very painful to see him in such misery. Thank you for any suggestions.

He's going through a lot right now. He's alone in a strange place after major surgery. No wonder he's blue.

One thing I disagree with is that he needs to go back to his colony. He's gotten semi-used to being inside now and doesn't have his brother. He might not make the adjustment back to being outside. So many things can happen to colony cats, I think it is better to continue to socialize him and find him a home.

Let him grieve. Leave out food and water and allow him to come around at him own pace. If you see him getting, sick, though, take him to the vet. If he hasn't been checked for the parvovirus, do so. I would also talk to the vet about his lack of appetite. He may have something else.

Thank you for helping them. You're doing him a tremendous service. In a way, the original caretaker's refusal to take him back may be working in his favor. Fingers crossed he becomes a happy, contented house cat.

I agree with Ondine--I wouldn't return him to the colony at this point, either. It's too bad the colony caretaker is blaming you for the death of the other cat--she should know better than to lay a guilt trip on you for something that is not your fault and out of your control. I'd advise you talk to your vet--s/he may want to examine the cat again. Are you able to handle him at all? If so, you could try to syringe feed him some baby food or soft wet food. If you syringe feed, you might want to get some YouTube tips on it because you don't want to accidentally get food into his lungs.  Cats grieve and take time to adjust to changes, but they're resilient, and if you hang in there with this kitty he will likely come around in a few weeks and will be happy to have a home of his own.    

Have you tried anything with tuna and egg? How long has he not really eaten anything? I do agree with the others, this is quite a lot for a kitty to go through. I don't know if you did or would have been open to doing so but when one of mine passes on, I hold a little wake for the remaining cats to say goodbye and come to terms with what has happened. It might come in handy if you did so the next time so the next one would have the time to say goodbye. Have you put any food under the sofa with him? He might eat under there. You might also have luck with leaving a raw chicken leg, not cooked and see if he'd eat that. He might not know the other stuff is actually food. I had one like that before.