First off, just remember this is his wedding too. Yes, tails are a bit formal for an afternoon wedding. With that said, if your whole wedding is going to be a bit more on the formal side (lots of attendants, full length gowns, etc.), no one will notice. However, if he wears tails they all should. Or you could compromise. Has he ever thought of wearing a cutaway? It's a bit more formal than a regular tux but not as formal as tails (it is also the most appropriate for a traditional afternoon/early evening wedding) I do suggest not taking him to the store and having the sales person "set him straight." This tends to discourage his involvement all together. Good luck!
"Marriage isn't fifty-fifty, it's one hundred-one hundered." -Dr. Phil