The most important one is the lake of motivation. It may be the subject matter, peer pressure or simply getting your attention! Sometime just a power struggle between two parties (parent / child; teacher/child)
See how his friends are doing and also talk to your son in a friendly manner putting yourself into his shoe.
Students that struggle, do so for a number of reasons. You should contact his school guidance counselor to find out from their perspective what the nature of the problem is and what they and you can do to support him.
There are programs available, depending on his needs, that may make a difference.
It is good that you are expressing a concern....now take it to the school staff for their support.