All three of those exams are quite passable. Go to Amazon.com and buy a copy of the CLEP Official Guide and the Princeton Review Cracking the CLEP books. The Official Guide has samples of all the CLEP exams. The Princeton Review book only covers the "general" (6-credit) exams but it covers them more thoroughly, and the sample exams are a little bit better (in my opinion). It also provides explanations of right and wrong answers. Both books give a good indication of how prepared you are for the real exam. Let your son take the practice exams, you score them for him, don't tell him which questions he's getting wrong until he's scoring over 50%, and he's ready to take the real exam when he's scoring over 70%.

-Gary-