CAT SCRATCHING UP MY HOUSE!!!!!!!!!!!

My 3 year old cat has taken to sharpening his claws on the trees outside ( we live in the country) since the day we got him, which is fine except for in the winter when he spends most of the time in the house. He then takes to (as discreetly as possible) sharpening his claws on my bedroom walls and the occasional door frame. Last winter I clipped his front nails but that had to be done weekly. I did purchase a scratch post even put catnip all over it( he loves the stuff ) and he refused to have anything to do with it. HELP!

Thanks in Advance

get him an upright log to scratch on - it resembles the trees he knows and loves.

Coat the areas he normally scratches with aluminium foil. When you catch him scratching, pick him up and take him to the scratching post. When he uses it correctly, praise him to the skies!