I agree that it would look weird if he was more dressed up than the groom and i think the pics would look a little weird too. I know when i have to break something gently to my dad, i usually just let my mom do it. I'm a chicken. But if your brave, i would just tell him that although his tux is really nice and he would look great in it, you don't think it would be appropriate for a DW and just suggest he wear a linen or tan suit.