At the risk of getting all "ABC After School Special" on the thread, if the OP is serious about trying to do what's right for the kid...
With all due respect, your post was a nightmare to read. Normally, I don't give a rat's ______ - it's the internet, not Honors English. But when someone with such poor writing skills asks whether or not it's a good idea to pull her brother - who's ALSO a poor student - out of school for a Disney trip, my immediate reaction was "Hell, no, chain him to the desk and make sure he's a straight A student before he ever is even allowed to see daylight, much less a Fastpass attraction! It's the only chance he has!" But then I put the snark aside and thought, maybe this could be a good experience for all of you...
So what I'd suggest is, make the trip, but be on him to do his homework like white on rice every day between today and the day of the trip. Slacking off will not be tolerated. Skipping school will not be tolerated. Make sure he does the work. Go over it with him. Learn it with him. And act like a mom that cares if he starts slacking off. Within a few weeks, you should be able to tell whether or not he's serious about being a student, doing what it takes not only to go to WDW but to make you happy in general. If you make it clear that his doing well in school is important to YOU - the person who is now probably THE most important person in his life - that's going to carry some weight. If you try to do the work with him (NOT for him), it would show some serious character and may influence him to work that much harder. Be involved with his teachers, make sure THEY know you give a rat's ***, and will lean on him to get stuff done. Because unlike the Ph.D. holder (who apparently doesn't know how to spell "ma'am"), I feel that working hard in school is important, even if you learn nothing important, because a good school and a dedicated parent (or parental figure) can help a student learn how to think, critically, logically, analytically. The brain's a muscle, exercising it's important.
So show him you're serious about his education, even though you'd also like to have some special memories with him that school can not provide by going to WDW. Make him work hard and help him out as much you can and the best you can. And keep it up during and certainly after the trip. Hopefully that will yield some obvious dividends, now and going forward.
All this is written under the assumption that you really wanted advice, and not just people giving you an okey-dokey, to a decision you already made, to make you feel better about it. If my assumption is wrong, please disregard my well-meaning, earnest, heartfelt advice and GO! Have a GREAT TIME! School Schmool, when you can go to WDW nothing else matters! :wave: