Tree ducklings actually do have those markings:
Heres a blog with photos of really young whistling ducklings:
http://flyingmullet.blogspot.com/2009/11/black-bellied-whistling-duck-brawl.html
Here's some slightly older ones with mom:
http://www.flickr.com/photos/billtex48/1698772589/
There was someone here who had a black-belled whistling tree duckling here on BYC and it looks similar to that kind, except they're native to Florida and areas south of there. However, I did look up tree ducks and found that some people do keep them in Europe as pets/poultry, but I'm still not...
I think it's a tree duck, possibly from a wild mama. I don't know what kind exists in Belgium, though. The bumps on its bill may be a sign of an abcess. Also, it doesn't look all that great, healthwise.