I know I'm a bit late into this thread, but, from an IB teacher's perspective, here are the traits I think make a really successful diploma kid:
1) the ability the make connections between concepts, within and between courses (for example seeing the connection between the math they are learning in math class and the science they are learning down the hall, or the novel they just read in english).
2) time management - yes, there is a lot of work, BUT it is manageable if the student does work everyday for every class - note, I am not saying that they have to do hours of work for every class everyday.
3) interest in a variety of topics/courses - some of my worst IB students were ones who had already made up their mind that everything that was not specifically within their interest zone was useless and they fought having to learn it the entire way
4) having the ability to accept that the grades they get may not be as high as they have always gotten
5) those with supportive parents who help them with time management and stress management
6) Most important of all: those that are most successful want to be in the program because they like the ideals of the program, not that they necessarily like all of the work, but that they appreciate what the over arching ideals of being a global citizen are all about.

I have taught IB for 11 years, and am an examiner for them, so I have a pretty good insight to who walks away with a diploma and college credit and who doesn't. Not only that, the research is there to show that IB students, whether they get the whole diploma or go for certificates, do better in college and are better students for their experience.

Which brings me to another point - does your school allow students to go for certificates rather than the diploma? If so, then I would say have him start out as a diploma candidate, see how he does for the first year of the program and then if things get too bad, see if he can switch to certificates in just the courses he is being successful in. (Yes, I would give it the entire first year. I know it is a long time, but the difference in kids from when they walk in the door as scared/eager year 1 students to when they leave at the end of the year is amazing, and most of them have figured the system out by then as well as whether or not it is the program for them.)

I hope this information helps.