My boyfriend is Michael, but he's been called Mikey since he was a kid. He'll be 27 in three months, and he still gets called Mikey. I think it's a bit juvenile, but no big. Some people call him Mike, and some call him Michael. It seems like the people that have known him since he was young call him Mikey, and then, people who have met him recently call him Mike because that's what he's been introducing himself as. Others call him Bart, but that's a long story. :)
I think that if he constantly told people Andrew, they'd call him that. At first, it would be a pain to have to correct people, but that happens with a lot of aspects of names all the time, so I don't think it's such a big deal. Strangers and random people will inevitable call him Andy (like they do with Abigail / Abby), but the people who matter will get it right, so I wouldn't let this deter you.