"Doing taxes with Turbotax, if he did 3K into a traditional, he would get back $600 more on his taxes for 2015."
Since there isn't a 20% tax bracket, I assume state taxes are also involved. When you contribute $3,000 to a Traditional IRA, it's tax deductible now but you pay during retirement. The $3,000 tax deduction gives you $600 saved on taxes. For someone in the 15-20% tax bracket, a Roth IRA lets you "lock in" that tax bracket for retirement. You use after tax money (the W-2 income you already paid taxes on) to fund it, and never pay taxes again. I prefer Roth because it's more "what you see is what you get".
I'd suggest trying to represent both sides to your son, and let him decide. The awareness and responsibility of making the decision are more important than the decision itself.