I do think having the second hole will make a difference - at least, it did for our ferals.
And he probably is getting fatter - regular food, probably higher quality food - and though you may not think he's warm, he must be warmer than he was - so he's not expending the energy to stay warm. Also, he's not getting much exercise now because he's found a good spot!
There's a woman on TCS that had an older feral rescue - not a stray. The kitty was free to come and go from her laundry room (had access to the outside, but not the rest of the house because she's got a kitty that will not tolerate other cats in the house). He lived in her laundry room, and slept in the dryer for eight years. Some cats do not want much territory!
Also, to help Caspar become even more comfortable around you, get a shirt that you don't mind giving up or not having for a while really good and sweaty. Put this under his food dish.
And if he's in the garage - just spend as much time in there as you can! Drag a chair or something out there and sit and read (if you can stay warm enough). Just being around him without trying to interact with him every time will help gain his trust more quickly.
Also, to help him get liquids, ask your mum to get some chicken for you to boil. To help our ferals warm up in winter, we would boil chicken in water (nothing else added), and we'd put out warm (not hot) chicken water for them to drink at night. We'd leave water out for them to drink - but it kept icing over. We fill the bowls with warm water every morning, but they LOVED that warm chicken water at night.