O.K., I have not read every post but here's my input sorry if I duplicate someone else

. We still don't appear to know exactly if the trip is 8 days or the number of school days missed is 8, which I think makes a big difference. Also is he struggling in everything or just some things? Is he failing or just not where he was compared to last years grades, which could simply be a case of adjusting to a different type of enviornment and several teachers a day instead of one. That being said, I think you need to do a combination of things.
Sit down with him and talk to him about why he is having so much difficulty this year. Don't lecture and point out where he is messing up, but really let him talk to you and listen to what he is saying without judging him and making him feel like he is a failure. In middle school I think is when our children's phyche(sp?) is most fragile and we need to build their confidence not tear it down. He may have the answers himself about what to do to help him.
Then I would have a meeting with your son and the teachers in the classes in which he is struggling. Discuss where the difficulties lie and what all of you can do to help him. It may be an easy solution or he may need a tutor, but until you talk about it you won't know. Again this needs to be non-threatening or you will lose him. I remember in 9th grade I had a science teacher who was a real jerk. We were not allowed to take the text book out of the classroom because there weren't enough for everyone taking the class. Our reading assignments had to be done in class and we then had to answer the questions after we were done and turn them in before class was over. I am sure I have dislexia(though I was never diagnosed) and to this day (I am almost 40

) I couldn't read a text book to save my life. I was really getting a bad grade in this class and my mom asked why I was having so much trouble. When I told her she went to the teacher (to this day I still HATE

that man) and explained the situation and from then on I was allowed to take the book home and turn in my questions the next day. My grade got better. I have long since learned what kind of learner I am and discovered if I payed close attention in class and took meticulous notes I got good grades. This discussion with him and his teachers may help you figure out his learning style and things will improve

. Also talk to his teachers about your plans and get their input, they may have a way for it to work out for you all

.
Lastly I would use it as a reward. Tell him if he works hard

and brings up his grades, and his teachers feel it can be worked out, that you'll be able to go on your trip. You have until April to make your decision. If no headway is being made you may have to pass this time, but let him know that you are not using it as a punishment. Let him know you are trying to help him make his first year in middle school a success and this trip will be a detriment to that success and you will simply have to do it another time. Make sure his siblings understand this and don't allow them to make him feel any worse than he already does and enlist them in helping him so you can all go. If he needs your undivided attention to help him with his homework for 1 or 2 hours tell them to help by keeping themselves busy during that time with either their own homework or another quiet activity. If he just needs a quiet enviornment get them to help locate a good room in the house for him to use and then everyone leave him alone and stay away from this room so he can concentrate. If you make this a family effort

he is more likely to be successful and they are more likely to be sympathetic and understanding if things have to be postponed. If his teachers feel a few days would be acceptable if he improves over the next 2 months, then a good compromise may be for a shorter trip. It may not be the origional plan but at least everyone gets to go

and you just make the most of your shortened vacation.
One more thing. Tell him you love him

and whatever he needs for you both he will get. Let him know you support him in his efforts to turn things around, and that as long as he works hard

you will be happy even if the grades aren't what you are hoping for.
Good luck. I hope you are able to make your trip.