Schedule a meeting with your sons teacher right away and spend some time beforehand preparing a list of questions to ask at the meeting. Some questions might be the following: What areas is my son struggling in? What is your impression of what is getting in his way? What can I do at home to help him in these areas? What resources are available at school to support him?
If your son's teacher is not able to provide specifics in terms of what areas he needs to work on and what you can do to help him graduate to the next grade, I would consider requesting an evaluation. The evaluation would examine whether there is a discrepancy in his ability and his achievement (which suggests a learning difference or disability of some kind). If he has some kind of learning difference, he would qualify for special services (things like tutoring or speech therapy) to help him manage the demands of the school day.
To make a formal evaluation request, you need to write a letter (see a sample below) and send it to the school. They are legally obligated to respond to your letter with a plan for assessment or a rationale as to why an evaluation is not appropriate at this time.