I have had both declawed cats and cats that are not declawed.

Looking at it from my point of view, I do think cats need to be kept control of when it means the health of your furniture or other things the cat may decide to destroy. However, declawing also meant the cats did bite more, they were not defenseless at all, but thier biting became a bit more problematic. I have tried the caps that was suggested by Ivy, they are great, as long as you remember to check them often to catch them when they do come off. As long as you catch them and replace the caps, you wont have torn up furniture or anything.

And as others have already said, the marking your cat is doing may or may not stop at his age after being nuetered. Really, it is best to spay or nueter an animal as young as possible to avoid this type of situation and many others that can arise from your pet not being fixed.