Hi! I am a Licensed Veterinary Technician and have worked in 2 different cat hospitals for well over 10 years. I'm looking forward to assisting you!My goodness! What a situation to have! It sounds like he is quite feral to be this scared to climb up the chimney and to still not be coming around after a month. I am glad that he is at least coming down to use the litter box and eat though. It is very important to be able to close him off from getting up into the chimney though since that can be unsafe for him. Since he only comes down when you are not present, the easiest way to catch him may be to use a humane trap inside your house. Here is some information on what they look like and how they work:http://bestfriends.org/Resources/No-Kill-Resources/Cat-initiatives/Helping-Community-Cats/Humane-Trapping-Instructions-for-Community-Cats/I would certainly be careful of trying to pull him out of the chimney because he probably will bite you just out of fear or he may climb up further into the chimney. Often times you can borrow a humane trap from your local SPCA or even your vet hospital. The trap will harm him in no way, though it of course will frighten him, but it sounds like he is already quite frightened anyways. You will want to place his food inside of the trap near where you would normally place his food.
Once he is trapped though, you should be prepared to close up the chimney are so that he will not be able to climb up there again. My concern though is that he may find another unsafe area to hide....so it is very important to also give him another option similar to the chimney area where he can feel comfortable, but be safe. Perhaps a cat tree with a great hideaway area that is in a secluded area of the house. Feral cats can act quite unpredictable when scared and do things such as climb walls, etc. It may be good to have him secluded to a small area of the house for awhile with this good hiding spot and his food and litter box nearby and allow him to get comfortable with this area and more comfortable with you before he can roam the house more. You may want to try adding some catnip to his "safe area" as well and give him some blankets to hide under. I would use some Feliway plug ins too which are similar to air freshener plug ins but release synthetic happy cat pheromones, which have a calming effect on stressed out cats. http://www.feliway.us/Any positive interactions that you can do while in the same room with him will be good even if he doesn't seem to be reacting to what you are doing. For example, getting out a fun cat toy or cat laser and playing with it nearby, although he may not come out to play right away, he will at least associate you with something positive. Or even just coming in the room and leaving some treats or opening some canned cat food, he'll start to associate you with positive things.I hope all of this has been helpful and that ObiWan can be more comfortable soon!Please do not hesitate to ask any further questions. I aim to provide you with the best service possible. If you feel I have done so, then an excellent rating is greatly appreciated! If not, please let me know how I can better assist you. Thanks! Best Regards,FelineMed