As a former lifeguard I can say that unless your son is extremely muscular his gender should make little difference in whether he can float. Girls do tend to have more body fat (once they hit puberty) so they may float a little easier, but unless he is really muscular (pretty rare for a 12 year old) that is probably not the issue. I taught swimming lessons and found fear and body tension to be the biggest deterrents. Sometimes something as simple as putting goggles on helps relax the person so they can float. Who is trying to teach him? You might want to try a different instructor.