Answer: It
sounds like your young son's participation and his performance in football
is much more important to you than it is to him. For example, you describe
yourself as "seething inside when a smaller more aggressive player
beats him at the line of scrimmage."
As a 10 year-old he should be
playing the sport because he enjoys it not to fulfill some high
expectation, desire or fantasy you may have. My advice to you is to back
off, let him develop some confidence and skill, help him with techniques,
etc. if he requests help but don't belittle him if he doesn't
perform up to your standards.
Kids develop at different rates
especially when it comes to aggressiveness and football specific skills.
I've known a number of young football players who were pretty good
athletes but rather timid at the start, as well as several kids who were
big, slow and clumsy or not athletic initially. With positive
encouragement and time to develop their skills and mature, they stuck with
the game and after several seasons became solid players who loved contact
and continued to play football in high school.
As for your son, let him continue
to play as long as he wants to and as long as he enjoys the game.
Encourage him to always try to do his best but don't let your high
expectations undermine his confidence or self worth.
I hope this helps.