I think the best way to respond to him is to appeal to the issues that are important to him. He seems to care about fiscal responsibility -- so, ask him, who was more irresponsible, W or Clinton? Remember that Clinton actually balanced the budget for a brief shining moment. Bush is giving us record deficits, each new year smashing the previous record set the year before.
If he thinks that McCain will start bringing us back towards balanced budgets, he's crazy. McCain is committed to American Empire, which costs at least as much as the social programs the Dems want. Both parties are committed to massive deficit spending, the only question put to the voters is, "How you you want your crushing, country-destroying debt served to you?" The Dems will serve it with a smile to the mothers and children and elderly, as well as giving fat cash to Wall Street. The Reps will take a serious approach, gripping their M-16s and saying the deficit is necessary to protect us, while they shovel money at Haliburton and Blackwater.
Neither party gives you the option of "Hey..um.. I don't really want my country to be destroyed.."
Seriously.. if his main issue is fiscal responsibility, this is an easy one. Obama and McCain are equally irresponsible.
But, if he believes this crap about terrorism, that the war in Iraq is necessary to protect us, then it is more difficult.