Hairless rat under treatment of baytrill loosing wight

Question
Several weeks ago (1-3 weeks)I took my male hairless rat Pinky to the vet because he had a bit of red discharge coming from his eyes and his breathing was a bit labored. Upon the visit I was told that he had dry eyes and that more than likely his myco had a flare up and caused difficult breathing. I was given 100ml of diluted baytril to be given every 12 hours and eye cream which was to be given every 8 hours, then was told to visit the vet once the baytril was gone and it is about that time.
So far Ive seen a bit of improvement but since I was afraid of his cage mates making his situation worse and stressing him out I moved him to his own cage the day we started medicating him. Ive given him his lab blocks, mix food, water bottle and his hammock and blankets to make him feel at home.
However now it seems he is sightly smaller than before, though he has always been the smaller of my three male rats his spine is a bit more predominant than usual and near his hind end he seems to be getting tan spots and he smells sort of..sour? In any case I am worried with it being winter and all that he may be burning off to much of his fat or at least the little he has. The house I live in is rather old and the heater does what it can to keep the rooms warm but since the severe temperature drop the rooms feel a bit colder than usual. Ive put warm water bottles in his hammock with him and even filled a large bottle full of hot water and put it in the cage at night to keep him warm as possible.
If you could possibly tell me what I could give him food wise to help him gain weight and to also give him extra fat to burn I would be very appreciative, so far Ive read that high protein dog foods may help do the trick.

Answer
I would stick him back in with his buddies, it can be that he is expanding more energy trying to stay warm because he is getting cold, you can still give him his meds, but remember he could be lonely and stressed for being by himself and sick at the same time.  Put him back with his friends, continue his treatment.. just like a person the stimulation will help him heal faster too.