Cat pooping on expensive curtains!

Hello there
I've come here in extreme desperation. My cat who is almost a year old, who I was convinced was the most well behaved cat in history, is actually a lot more sneakier than I thought. All along I have thought he's been using his liter box and never messing anywhere else. Well recently I have discovered he has a secret hiding place he likes to go. I'm 21 years old, and I live with my mom. I had to do a lot of convincing to have my little love bug come and live with us and so far she really hasn't had too much to complain about because he is not destructive at all. But I found out he's been pooping behind her expensive silk ballroom curtains in our dining room. We never much use our dining room so it took me a while to figure this out, but if she finds out he ruined them he will have to leave which will break my heart. I need any advice possible.
When I had moved the curtains the other days, enormous amounts of poop were stuck to, inside, and gathered all behind her curtains in the same area. He for some reason likes this particular little area because it is not behind any of the other curtains. I have to keep his liter box in my bedroom, because she does not want it to be seen in the house. I had picked up all the poop, but days later I came back and there was more.
1. What can I do to make him stop this as soon as possible? Keeping him out the dining room is impossible. There's no other areas in my house besides there, and the liter box that he messes, but I need this to end as soon as possible.
2. If I do somehow manage to convince her to let me keep a second liter box in the dining room when company is not over to see it (which she may or may not even allow) would you think this would benefit in any way, or would he just still continue to use his new found favorite area?
3. What on earth can I do to get the bottom of these silk ballroom curtains that I know my mother paid an arm and a leg for? She loves them so much and it's going to break her heart if they're destroyed. The stains look like they would be able to come out if I were to be able to wash them if they were any other kind of fabric, but I know this is not the kind of fabric that you can just carelessly care for.
Any advice or suggestions that you may have would be very appreciated. He really is a good cat, but I need this to stop as soon as possible before he is put out. Thank you so much.

First- welcome to TCS 

Cats will not use the litter box for a number of reasons. It is being scooped regularly? Has your cat been to the vet recently? Litterbox problems can sometimes be medical problems. if a health related issue has been ruled out, it could be that your cat does not like the location of the box, the type of litter used or the box itself. Some cats are very picky about these things, others are not. Below is a link to a very helpful article from this site. The rule of thumb is 1 litter box per cat plus one extra. 1 cat- 2 boxes, 2 cats- 3 boxes, etc. 

http://www.thecatsite.com/a/how-to-solve-litterbox-problems-in-cats-the-ultimate-guide

As far as the silk curtains are concerned, I think that the only way to properly clean silk is to have it dry cleaned. I wouldn't try to put them in the washer but I will look around.

Here is a site that states that they can be hand washed in the bathtub

http://cleaning.tips.net/T008459_Cleaning_Silk_Curtains.html

Oh no! Naughty kitty.

The best thing to clean the curtains with would be an enzyme cleaner, which gets rid of the stains and the smell. I've never used one on silk before but they're usually very gentle. Maybe you could Google enzyme cleaners and see if they are suitable to use on silk, you could e-mail the manufactures and ask them. Other than that, could you contact your local cleaners and ask what they suggest?

If the cat has decided that's his spot you should put a litter box there. Even if you do get rid of all traces of smell he obviously likes that spot and will keep going back there. If it's a room that's rarely used you could keep it there and then just take it out whenever you have guests. maybe better to keep him in your room when you have company, just in case.