You can make medium carb by mixing the regular low carb and some of the high carb - if it's half and half then the carb count is the average of the two. He might not need the extra carbs, I just mentioned it in case you want to be really sure he's not going lower. Feeding him some of the regular low carb might be fine too. If you're worried about the other cat eating his food, is there a way you could put one of them in a different room for the day with his own food/water/litter? This might totally not be anything to worry about, since 105 is not that low and you likely only have a couple more hours before nadir, but it might give you more peace of mind.