Cats need things to climb and places to sit high up so they can view their domain. If he doesn’t already have a cat tree and scratching post, you should get him one. It doesn’t have to be huge but it should have both sisal and carpet to scratch on and a platform at the top where he can sit.
There are some good samples here: http://www.petplanet.co.uk/category.asp?dept_id=816
While you are checking out those trees, also take a look at the cardboard scratchers and activity centres. You may want to put a couple simple scratching toys in other rooms. Cats need to play hard, and if they aren’t getting enought play time from their ‘staff’, they make up stuff to do.
Try getting him a feather and wand type toy for playtime with you. There’s something similar to what I have called a Pouncer on the website above. Don’t let him play with it without supervision since many cats will try to eat string or yarn and that can be very dangerous, even fatal. Do play with him a couple times a day until he is literally exhausted.
We have a TDK member who makes some great cat trees, and I think she is in the UK. But I’ll be darned if I can remember who it is right now. Maybe she’ll see this and post.
If he isn’t deterred by water, try noise. Put some coins in a tin and rattle it vigorously while saying “NO”, when you see him going for the curtains.
Reward him with praise, pets and treats when he uses his cat tree. In fact, you can put some treats on the top level now and again just to remind him that the cat tree is a great place.
You might have to remove the curtains for a few weeks, but once he learns his tree is the right place to scratch, then he should leave the curtains and furniture alone.
Good luck. Do you have pictures? We love looking at kittens of all ages.