Does anybody know...???

Hi everyone,
My 17-year old son just was nominated by our congressman for admission into West Point for fall of next year. Getting into West Point has been my son's dream. We are not a military family, so this nomination is quite an achievement for him. Being rather naive about all of this nomination process, I have a few questions. Hopefully, somebody out there can help me. How is nomination by a congressman different from nomination by a senator? What further "weeding out" process will my son encounter? What percentage of those nominated are finally accepted?
THANXXXX!!!!