Help!! how can i start jumping my horse?

I have a 12yr 14.2 cob which i want to start jumping, but i have been in the paddock day day and ended up leaving in shear deperation as he knocked ever singal jump down. First i gave him a straight run up with plenty of room for a nice canter, thn whn tht did't work i cantered around the paddock and tryed it form different angles. I have seen him jump with different people so i know that he can do it! But he just chooses not to with me. I have tryed to do it so he followed my friend and her ID through but with no avail, he just ploughs through them. Yet if i take him out in the woods he may need a little persuasion but he will do it. I use a schooling whip although he can be quite responsove to the leg in the i tap him before he jumps..... but still no! i am really frustrated as i know he has it in him! i only set the jumps low to give him confidence. I know the person i bought him of so i know it is not something in the past that has scared him :S