Very few people in Belgium speak both French and Flemish natively. It's one or the other, depending on which side of the language border you live on. In Flanders you have compulsory French classes while in Wallonia you don't have compulsory Flemish classes, but the compulsory classes don't get you anywhere near native proficiency. His last name doesn't sound very Flemish so his ancestors probably moved to Wallonia at least a few generations back. To me it even sounds like a Flemish version of a French name. The parent should correct me if I'm mistaken of course, deducting where someone is from based on their last or first name can be quite tricky in my experience.