Your b/f is teaching your son to lie to the teacher and say that they did his homework assignment when they did not. (change the tire.) Now, your son does not know how to do something that's pretty dang important for anyone to know how to do, AND he's learned from the b/f something much worse -- it's ok to lie; it's ok to lie to his teacher; and it's ok to lie to an adult. And that means, it's ok to lie to him and to YOU!
BAD MESSAGE TO SEND TO A KID!

I'm a teacher. I see lots of stepparenting. He doesn't know how, *or* he doesn't care. I cannot say which it is; I'm not there and only have this one instance to go by. But, you've said alot.

I suggest family counseling for the both of you. It may involve your son later on. But, the two of you better get on the same page and soon, or you will have one confused boy.

And you do not say if your son has any learning difficulties or anything, but, a month's grounding for two bad grades *may* be excessive if his past performance has been similar. Again, not enough information to go by. But, every high school has daily report sheets that can help you and the school keep in touch with your son's academic progress. That could turn down the heat at home.

I wish you luck!

Peace!