i don't really have any good advice, but i'll offer my observations.
he sounds fixated and i suspect that if he does not attend at all then he will be complaining about it for a long time, and possibly blaming people around him for not facilitating his dating endeavours. the reason why i am saying that is because of your facetious comment about "ruining his life". i suspect he has said something similar to you, and it is often a pattern of blaming others for.... any perceived failure or disadvantage.
so... if he is friends with a girl she will be his best choice. she will be most likely to say yes to him if she is similarly dateless. randomly asking pretty girls is likely to fail. if you think failure is a good learning experience, it could be a good idea. but otherwise, i don't think there is much chance it will work. word travel fast, and asking more than a few people to prom starts to look really bad (and it will affect his confidence too).
it is possible to get a girlfriend before prom if it will be held at the end of the school year. as he gets closer to the prom, there will be girls that are still waiting anxiously for a date and they will probably be open to general dating at that point. talking to other guys will allow him to scope out which girls are still single and waiting for a date, as some of them will know the latest gossip.
sometimes, it can be tough to find a date if a person is a loner because often people pair off within a peer group. so if someone isolates themselves they will have a tough time knowing the dynamics of the group ( who has a date, who is likely to say yes, etc.).
looking sad and waiting for a girl to approach him is not likely to work. they will probably think there is something wrong with him and are likely to avoid him. if he was friends with a girl and looked sad, she might ask him what is wrong or offer a hug maybe, but that is about it. some girls like sad-looking guys, but the chances of finding a female like that who ALSO has the confidence to ask him out is extremely remote.
i didn't go to my prom. i didn't really see the point. a lot of people don't attend. check out this thread:
http://www.wrongplanet.net/postt160507.html
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