Newbie with Clep Biology Question

[Mom23boys]

Newbie here, just discovered the world of CLEP and can't believe I never knew about this option to earn college credits for little cost or even free. I have two sons in college already, all gen ed classes have been taken, but I have one son in high school. I'm researching the possibility of him taking CLEP while still in high school. 

This year my son is taking Honors Biology and is doing very well (95%). They use the Campbell's Biology Concepts and Connections textbook, which is very in depth. His tests are very difficult and he spends many hours studying, which pays off with his good grades. By the end of the year, he will have only covered 1/3 of the textbook though. He will take Honors Chemistry next year, then move on to Physics. This will be his only Biology class.

Do you think it would be a good idea for him to purse the CLEP Biology exam this summer since he will have so much material fresh in his mind? By the time the class is over, he will have learned most of molecular and cellular biology (33% of CLEP) and a little bit of population ecology and organismal biology. Still, my research tells me that this is a difficult CLEP exam.

Thoughts/advice/feedback is appreciated.