Yes, gay students should be allowed to bring same-sex dates to school dances. Public schools should not be allowed to discriminate based on sexual preference. If these students are gay, it won't be a shock to other students who have probably been around them on a regular basis. If it is uncomfortable or objectionable to some parents, then those parents should sponsor a similar event themselves at another location and make it clear that it is for heterosexual couples only.
There are many groups of friends all of the same gender, who attend homecoming together. As long as he is not trying to make homecoming court as queen, there really isn't much the school can say about the situation. However, he could have just as easily attended with the boy as friends, and not felt the need to draw attention.
There is too much of a "sue" culture currently spreading around the world, especially in the USA, which has resulted in people filing lawsuits for ridiculous reasons. Yes, the boy may feel discriminated against, but it depends if the school already has a rule in place regarding banning homosexual relationships etc. If that is the case, then fine, he should look to change that. But suing the school for his own financial gain is not the answer. He should seek to change the rules in other ways.
Although we live in a society and world where free thinking and progressive attitudes are becoming the norm i still think that it is important that schools and organisations are free to impose their own rules and traditions. I certainly do not believe that a litigious strategy is the correct way forward.