It seems that these stories of unknown children pop up every now and then; and one should always view them with a substantial degree of skepticism. Frizzyperm makes an excellent point, that it took him only a few minutes to find a definitive source which disproves the rumor.
Quite frankly, if it were true, historians would have uncovered it by now and its authenticity verified. It seems to me that this Frenchman found a way to draw notoriety to himself by claiming to be Hitler's son. One can certainly think of better ways to get ones name in print.
As for HItler being part Jewish, there has been substantial speculation about this for many years. His father, Alois Hitler, was born to Anna Schickelgruber who was not married and refused to divulge the name of the father of her child. It was commonly known that she had been housekeeper for a wealthy Jewish family and rumors were about, even before Adolf Hitler was born, that the father of Alois was Jewish. Anna later married a gentleman with the surname Hitler who claimed to be Alois true father and gave him his last name; however the speculation continues. This speculation appears to have more basis in fact than that of an unknown son.