Look specifically for universities that offer biomedical engineering as a masters degree. These will have professors involved in research with prosthetics, or at least research that has a medical basis. They tend to be a tight knit group, so if one can’t help you, they may know someone else who can.
I do not have an exhaustive list of French universities that offer this, but I do know of a few. I’d say start with École Nationale Supérieure des Mines de Saint-Étienne, École Centrale de Lille, and ParisTech at Paris Institute of Technology. I would very strongly put emphasis on finding someone local, and someone who knows what they’re doing. Anyone can print a model, that’s the easy part. Designing the model to accommodate your nephew’s needs, and then fitting it to him are not easily done.