When a child stops trying in school, parents need to remember that it is their child's responsibility to pass -- not theirs.
My 13-year-old is driving me nuts. He failed the seventh grade last year and is on the way to failing again. He's capable of doing the work -- he just won't. He's a good kid overall and never gets in trouble.
Kids sometimes fail a grade level because they have a fear of going on; sometimes because there aren't any choices (consequences) that mean much for them. Keep in mind that it is your son's responsibility to pass and it is not your failure or success, but his. Prepare him now with choices: "When you successfully complete seventh grade in May, we will take that trip to..."; when he asks, "What if I don't pass?" you can respond calmly: "Well, I hope that isn't your choice, but, if it is, then you will have to take summer school for two months so that you can do better next year." Of course, talking with teachers and the school counselor can always provide other alternatives and plans to help your son succeed.
You don't have to spell out what he will miss because of summer school. He knows! Continue to encourage and offer to help, but don't carry the load. Believe your child can and will make the choices that are best for him.