Hi - biology is usually a high school subject -- could he be taking it too soon? Or is he taking a more general "life science" course, often part of the middle school curriculum?
It is usually counterproductive to force or push him to study more. What I'd recommend is trying to increase his interest in the subject. One possibility is to do some nature walks with him and call attention to the life forms you see on the trail. If he is a photographer, you might encourage him to record some of the beautiful specimens you discover (and don't forget to -carefully- look under logs and rocks for salamanders, etc.) Or, another thing you might do is to take him on a visit to a natural history museum if there is one near you. You might subscribe to a magazine like National Geographic or Popular Science and share some of the life science stories and photos with your son.
You might also talk with his teacher about his struggles, and s/he might have additional insight into his issues.
I teach HS chemistry and I have a daughter in 9th grade Bio. She absorbs everything I tell her in Chem, but also started the year struggling in Bio. "studying" meant looking over the material, making flash cards, and re-reading notes: it just wasn't staying with her. Then we tried just talking about it. I would sit with the book, ask her some open, general questions, and help her connect the info. We spend 10 minutes a night talking about what they did in class, what material is in the chapter, etc. She is now excelling, and we still spend that 10 minutes just talking through the material. Because it is spread out over numerous nights, it doesn't feel like studying, and she is able to keep the material staright.