Hi frazzlenz, just my thoughts...
I am living in Germany and my 2 eldest dc are in local school. We moved to Germany 2 years ago, and the boys spent 1st year in International school, as we weren't sure if we would stay here.
We did used to live in Switzerland so the boys could speak some German.
Anyway, I was not at all impressed with the IS. In our experience, the standards were low and they didn't cater at all for the brighter kids. My dss, who are both v. good at maths were doing work easier than 1st class work in Switzerland. It was astonishing. Yes, the facilities were great, but that's where it ends imo. Plus, here anyway, most pupils at the IS lived scattered far and wide from the school, so there was v. little contact with kids after school or at the weekend. School holidays were v. lonely and difficult. And making friends with the local kids was nigh on impossible.
We decided to stay here, and moved our dc to the local school. In only a month they had improved a lot. It has been hard in some ways, but even the social side more than makes up for that (ds1 is 10 btw) They have friends calling round daily. You can't put a price on that.
I agree with the poster who says getting an au pair would be great if possible, but wouldn't rule out the language course completely.
Oh, and finally, I have a friend here who moved from the US with her 12 yr old dd. The girl didn't have a word of German. She started in the local grammar school. Her mum really worked hard at it - organising tutors, homework help, holiday courses etc as well as lots of social stuff (a baseball team which seems to take the entire family's spare time). She also had only German tv. And the girl herself, who is very bright, of course,had to work hard, but she did it, she has done really well, is fluent in German, is thriving, has lots of friends. She is a real example of how it can work (with commitment and hard work)
Imagine if you are staying long term in Belgium, and you decide out of worry to send ds1 to IS, but the other 2 to local school. Before long they will be fluent/bilingual. He won't be. WOuld he resent the gift of a second language that his 2 siblings have that he wouldn't? I suspect yes. Also, if you do decide to stay long term in Belgium, he won't be in a strong position. I know he's only 11 now, but give it a few years and he wants to leave cos he doesn't speak French, but everyone else does and wants to stay...?
After our experience in the IS, I'd say if you're only planning on 1 or 2 years maybe, but otherwise, grasp the nettle