My youngest daughter was a trial for her teachers, beginning in kindergarten. They wanted her evaluated for ADD, ADHD and whatever else they could think of, because she had a short attention span and couldn't sit still for long periods. I refused the evals and told them to deal with it. She went from a pretty much unstructured day (we'd do whatever then move on to something else and then something else as they got bored..) to a full day of structure, how did they expect her to respond?
She struggled terribly with spelling and math, so bad that in 3rd grade her teacher called me in to discuss it and her "attitude". Poor dear was in tears as she asked me if my daugher hated her. Goodness no, I reassured her, she is always talking about you and the things they do in class. She was and always had been a strong willed child, most days it was like taking a bull by the horns!
This wonderful teacher was very confused by how poorly DD was in spelling and at first she suspected dyslexia, but performed her own little test and discovered how well she could read (we didn't have a TV for years when they were little, but we did have library cards!!)and understand what she read when given 7th and 8th grade books, yet she couldn't spell... IN, AT, ON, THE... nada, nothing. We studied with her for hours, tried eveyrthing, but nothing seemed to help.
I agreed to the tests... the conclusion? She did great in the subjects she liked but math and spelling were not important to her and there was no way to make it important to her. They did not want her to become overly discouraged and come to hate the subjects so they decided that the best thing for her would be to grade her on a curve, against herself, not the class. It worked great... she did improve quite a bit that year, but was never going to be a great student in spelling or math, but as long as she tried she got credit. Worked great until we moved from Colorado Springs to Illinois and she had the teacher from He!! who when asked if she had read the test results and recommendations said yes and it didn't mean anything to her. Her children had to EARN their grades and there was no curve (this was 4th grade). After that meeting I told her I didn't care about the grades, only that she give it her best, which she did, she studied hard and continued to show improvement.
My DD will be 34 next month and still isn't the best speller and not so great in the math dept either but she attended college for a year and pulled A's and B's before going to work full time and becoming a mother.
I offered this because sometimes there are other reasons for what's happening and other options available to help them through the rough spots. Hopefully between you and his teachers you can figure something out. If he's passing with C's I can't see holding him back, and grades don't mean everything. If you are working together with him and he's trying and there's no underlying cause, then I wouldn't be overly worried. Hoping for the best for your meeting.