Do you have to be born really athletic or can you become it?

I'm asking this question because my younger brother who's entering the 8th grade wants to play football but he hasn't played sports since probably the 5th grade...he is really fast, he kind of has my dads genes when it comes to athletics (my dad was a really good athlete in high school) but he is very small and undersized for his age..my parents keep telling me not to push him to play contact sports like football because he is still undersized but i keep trying to tell them that a) being undersized has nothing to do with how good of a player you can be and b) if we waited for him to magically grow a foot he'd probably be in college and besides that who knows he may not grow to be over six feet...am i wrong or right about this? he thinks he wants to be either a kicker or a WR but he's afraid that the coach will bench him because of his size and weight..our coaches do kind of suck there at my brothers school, their the kind that'll put their own sons and family friend's kids in before the rest of the kids, no matter how much they suck..i know if my brother tried he could become decent but he isn't just athletically gifted, he's had to work hard at sports even on a little league level just to be okay at it..do you think he'll have what it takes or no..? he's about 5'1 or 5'2..thanks in advance!