My first question for you would be: Do you check your child's classroom work (if it's sent home with him) and homework every night and review it with him? If not, then that's the first step. The second step is to ask the teacher to see ALL of the work that your son has turned in. Was it turned in on time? Was it corrected? Did your son ask for additional help if there was something he didn't understand? Teachers make mistakes sometimes in grading so double-check. My sister is a teacher and one of her parents asked why she was not informed that her child was failing. My sister's reply: Did you ask about your child's progress? Did you ask to see the work that your child has completed or not completed? It is the ultimate the responsibility of the parent and the child to monitor the child's progress weekly if not daily. You as a parent have the right to know how your child is progressing. And ask again for all of the grades; she does not have the right to keep that information from you.
As a former spelling bee champion, a Spelling course is a complete waste of time. You learn to spell (and write for that matter) by reading so supplement your child's learning with additional reading material. There's always the public library and some of the books are available for download as an e-book. The best way to learn how to spell is to visualize the word and spell in syllables. Also, if the spelling tests are given orally, have your child's hearing checked. It may be that he can't hear the words correctly hence the reason for the misspellings.