Hi, I agree with the other answers. In the mean time try leaving a box with straw in it as straw is better for keeping animals warm. Could you manage to put a collar on him with a message attached to any possible owner?
The Blue Cross should have been more helpful in my opinion, what if you are unable to afford to feed another cat?! You may find that he is being fed by half the neighbourhood who feel sorry for him. My cat (unfortunately passed away now) had another 3 houses who he adopted, each one feeding him and letting him sleep in their homes. It wasn't until he was getting fat that we discovered this and had to put notices up requesting people not to feed the greedy, sympathy seeking little beast, bless him!
Anyway, good luck. Thanks for what you are doing for the poor cat. I hope he finds a good home with someone who cares for animals like you.