If one reads سبحان ربي العزيم with a ز, this will invalidate the Salah; therefore, if he cannot pronounce سبحان ربي العظيم adequately, he may recite سبحان ربي الكريم until the time when he is able to pronounce العظيم سبحان ربي correctly and properly. (Shaami Vol. 1 Pg. 494)
