Hello!
I'm only 22 and have absolutely no parenting skills, so take my advice with a grain of salt. I'm guess my answer stems mostly from the point of view of your son...

But, I feel that your boyfriend is being too harsh on your son. I dont know how long you have been with him, and I can only assume it's been a decent amount of time since you live with him. But, in my mind, your son is your son, and his grade dont affect your boyfriend or his home. If your son was bringing home drugs, alcohol, or friends that were bad influences, or was some how misbehaving towards your boyfriend or his property, then I would agree that your boyfriend would be allowed to discipline your son. In my mind, grounding someone (especially for a month!) for doing badly in school is sending the wrong message. You want to encourage your son to learn because he wants to, not for fear of punishment. I think you need to talk to your son about the grades, and maybe figure out ways that he can do better in school. You need to make sure he's doing his homework. The fact that he's doing okay in the rest of his classes, though, leads me to believe he might be struggling with those subjects or teachers.

To summarize, I dont feel that your boyfriend should be able to ground your son for bad grades when they dont affect your boyfriend. It will build tension between them and isn't going to encourage your son to try harder. I believe you need to handle your son's grades however you feel appropriate, and your boyfriend should be supportive of you both.

I hope this helps!
Ro