OP - it is hard to see your kids struggling and we all want them to succeed so I totally understand your concerns. However, you will exhaust both of yourselves if you spend the next year navigating his biology class this way! Also, since your son is only in 9th grade there will be plenty of other courses that are difficult. I would try and do the following asap:
1) Have your son spend time on Khan Academy (
https://www.khanacademy.org/science/biology). It is a wonderful resource! There are videos online that he can watch about various subjects. It started out with mathematics only, but it has branched out considerably. Your son will be able to watch as many short videos as needed about whatever topic is needed.
2) Talk with the school about additional help for your son. They may have some sort of subject tutorials available. Or they may be able to offer some other type of guidance. Reach out to them to investigate the possibilities.
2) Try to find a college or graduate student who would be willing to tutor your son. They may have lower rates.
3) Encourage your son to go to see his teacher as often as is needed. If the teacher is insisting on teaching at a high level then he/she needs to accept that they may need to offer a little extra help. Also, encourage your son to ask questions in class. He needs to know that if he has questions then others more than likely have the same questions. His classmates will thank him!
4) Look online for some other biology periodicals (textbooks, articles, journals) that may discuss the topics in an easier manner. Sometimes the language used in certain textbooks is unnecessarily difficult and simply reading about the same topic with different language will make it easier for your son. You can either look to purchase them used on Amazon or look for articles that can be read online.
Pushing your son to advocate for himself and find the answers will only help him in the long run. Continue to help him, but start making him be responsible for the work. You and he can review things that are particularly difficult for him, but all of these resources should help him.
Good luck!